Water-absorbent resin composition and its production process

ABSTRACT

There are disclosed a water-absorbent resin composition and its production process, wherein the water-absorbent resin composition causes little gel-blocking and is excellent in the liquid permeability and liquid diffusibility and is high also in the absorption performances and further is strong also against the physical damage; and there are further disclosed a water-absorbent resin composition and its production process, wherein the water-absorbent resin composition has the following further advantages, in addition to the above, of involving little segregation of the metal compound and further having a dust prevention effect. One of water-absorbent resin compositions according to the present invention is a water-absorbent resin composition comprising water-absorbent resin particles obtained by polymerizing a monomer including acrylic acid and/or its salt, with the composition having a mass-average particle diameter of 100 to 600 μm and comprising water-soluble polyvalent metal salt particles and the water-absorbent resin particles that have been surface-crosslinked.

TECHNICAL FIELD

The present invention relates to a water-absorbent resin composition andits production process, wherein the water-absorbent resin composition isused for sanitary materials such as disposable diapers, sanitarynapkins, and so-called incontinent pads.

BACKGROUND ART

For sanitary materials such as disposable diapers, sanitary napkins, andincontinent pads, there are widely utilized absorbent structurescomprising hydrophilic fibers (e.g. pulp) and water-absorbent resins asconstituent materials for the purpose of absorption of body fluids.

In recent years, as to these sanitary materials, their highfunctionalization and thinning are making progress, so there is atendency toward increases in the amount of the water-absorbent resin asused per piece of sanitary material and in the ratio of thewater-absorbent resin relative to a whole absorbent structure comprisingthe water-absorbent resin and the hydrophilic fibers. Specifically, theratio of the water-absorbent resin in the absorbent structure is raisedby decreasing the amount of the hydrophilic fibers (which have a smallbulk density) and increasing the amount of the water-absorbent resin(which has excellent water absorbency and a large bulk density) as used.Thereby the thinning of the sanitary materials is aimed at withoutlowering the water absorption quantity.

However, the sanitary materials, in which the ratio of the hydrophilicfibers has been decreased and that of the water-absorbent resin has beenincreased in the above way, are favorable from the viewpoint of simplestorage of liquids, but rather involve problems in the case ofconsideration of distribution and diffusion of the liquids undercircumstances of actual use as such as diapers. For example, the largeamount of water-absorbent resin becomes a soft gel due to waterabsorption to cause a phenomenon “gel-blocking”, thus dramaticallydeteriorating the ability to diffuse the liquids in the sanitarymaterials. In order to avoid such problems to maintain absorptionproperties of the absorbent structure, the ratios of the hydrophilicfibers and the water-absorbent resin have axiomatically been limited, soa limit has occurred also to the thinning of the sanitary materials.

As means for preventing the gel-blocking to thus obtain thewater-absorbent resin excellent in the liquid permeability and liquiddiffusibility, there are known the following arts in which metalcompounds (e.g. metal salts, metal cations) are added to water-absorbentresins.

There is known a water-insoluble water-absorbent resin compositionobtained by adding water, containing a salt and/or hydroxide of apolyvalent metal, to a water-absorbent resin (patent document 1).

There is known a process for production of a water-absorbent resin inwhich a water-absorbent resin is treated with an aluminum compound inthe presence of a polyhydric alcohol and water, wherein the aluminumcompound is reactable with the water-absorbent resin (patent document2).

There is known a process for production of a water-absorbent resin inwhich a water-absorbent resin is treated with an aluminum compound and acrosslinking agent in the presence of a polyhydric alcohol and water,wherein: the aluminum compound is reactable with the water-absorbentresin, and the crosslinking agent has not fewer than two functionalgroups reactable with the water-absorbent resin (patent document 3).

There is known a process for production of water-absorbent resinparticles having the modified particulate brittleness, in which process,to water-absorbent resin particles obtained by heat-crosslinking ofsurfaces and their neighborhood of particles of a water-absorbent resin,there are added, after this heat-crosslinking, water in which aninorganic salt is dissolved in a concentration of 5 to 50 weight %relative to water and/or water in which an inorganic hydroxide isdissolved in a concentration of 5 to 50 weight % relative to water,thereby adjusting the water content to 3-9% (patent document 4).

There is known a polymer produced by a process in which awater-absorbent resin is treated with a polyol and a cation which is ina state of an aqueous solution and then surface-crosslinked at 150-300°C. (patent document 5).

There is known a polymer produced by a process in which awater-absorbent resin is treated with an organicsurface-secondary-crosslinking agent (except polyols) and a cation whichis in a state of an aqueous solution and then surface-crosslinked(patent document 6).

There is known a composition comprising aqueous-fluid-absorbent polymerparticles having been heat-treated at a temperature higher than 170° C.for more than 10 minutes, wherein the composition is remoisturized withan aqueous additive solution in the absence of an organic solvent or awater-insoluble and non-water-swellable powder after the heat-treatmentand has a water content of 1-10 weight % based on the total weight ofthe composition and displays an absorption capacity of more than 20 g/gunder 0.3 psi in 60 minutes (patent document 7).

These (patent documents 1 to 7) are arts in which the metal compounds(e.g. metal salts, metal cations) are added in aqueous solution states.As to these arts, because the metal compounds (e.g. metal salts, metalcations) are added in aqueous solution states, the metal componentsunfavorably permeate the inside of the water-absorbent resins, thusresulting in insufficiency of the effect of enhancing the liquidpermeability and liquid diffusibility to a degree corresponding to theaddition amount. In addition, because the metal components permeate theinside of the water-absorbent resins, there have unfavorably occurreddeteriorations of such as absorption capacity without load andabsorption capacity under load.

There is known a modified water-insoluble water-absorbent resincomposition obtained by adding water to a mixture of a water-absorbentresin and a salt and/or hydroxide of a polyvalent metal (patent document8).

There is known a method in which: a water-absorbent resin and apolyvalent metal salt are mixed together, and then the resultant mixtureis brought into close contact with a binder in the absence of a volatilealcohol (patent document 9).

As to these arts (patent documents 8 to 9), there have been problemssuch that: the dissolved metal salt causes binding between particles tothus easily form a strong agglomerate and, in the case where thisagglomerate is crushed by physical damage such as during the actualproduction or practical use, the absorption capacity under load isdeteriorated. In addition, there have also been problems such that: thedissolved metal salt unfavorably goes so far as permeating intoparticles of the water-absorbent resin. The case where particles of thepolyvalent metal salt having small particle diameters are used has beenremarkable for the aforementioned permeation. Because of thispermeation, there have been the same problems as the aforementioned.Specifically, the effect of enhancing the liquid permeability and liquiddiffusibility to a degree corresponding to the addition amount has beeninsufficient or, because the metal components permeate the inside of thewater-absorbent resins, there have unfavorably occurred deteriorationsof such as absorption capacity without load and absorption capacityunder load. In addition, as to particles of the polyvalent metal salthaving comparatively large particle diameters, no sufficient bindingforce between particles can be obtained with the binder, and thereforesuch as release or elimination unfavorably occurs, so that problems ofsuch as segregation of the metal compounds (e.g. metal salts) have alsobeen caused.

As to other than these methods, for example, as to a method in which awater-absorbent resin and a metal compound (e.g. metal salt) aredry-blended together, particles are mixed with each other. Therefore,there is a possibility of occurrence of problems such that thesegregation occurs to thus result in unstable performances of thewater-absorbent resin.

As means for preventing the gel-blocking to thus obtain thewater-absorbent resin excellent in the liquid permeability and liquiddiffusibility, there are known some other arts besides the above arts asfollows.

For example, there are proposed such as: a method in which two kinds ofwater-absorbent resins different as to water absorption performance areused (patent document 10); a method in which a composition containing acationic ion-exchange hydrogel-forming polymer and an anionicion-exchange hydrogel-forming polymer is used (patent document 11); anda method in which a water-absorbent resin having a highsurface-crosslinking density is used (patent document 12). However, theyhave problems such that the absorption properties are unsatisfactory asthe absorbent structure having a high water-absorbent resinconcentration or that the cost is high.

In addition, a water-absorbent resin which contains a large amount offine powder due to such as abrasion in processes for production of thewater-absorbent resin has a tendency to cause the gel-blocking.Therefore, there is proposed a method in which the water-absorbent resinis made to contain water in an amount of not smaller than 3%, therebyimproving the brittleness (patent document 13). However, there areproblems such that the absorption capacity is deteriorated, and that,when water is added to the water-absorbent resin, this resin swells tothus form particles having too large particle diameters. In addition, itis also proposed that a special stirring apparatus is used to reduce theformation of the fine powder in processes for production of thewater-absorbent resin (patent document 14).

In addition, there are known such as: a method in which awater-absorbent resin is mixed with a powder of an organic or inorganicwater-soluble salt (specific salt such as thiourea, saccharide, orcarboxylate salt), thereby enhancing the absorption of blood (patentdocument 15); a method in which a water-absorbent resin and apermeability-retaining agent (e.g. silica, alumina, titania, clay,emulsion-polymerized material, precipitation-polymerized material) aremixed together by a Vortex Mixer and then subjected to mechanical stressby such as Osterizer blender (patent document 16); a method in which awater-insoluble and water-swellable hydrogel is coated with steric orelectrostatic spacers (patent document 17); a method in which awater-absorbent resin having been crosslinked with a specific metal ionis used (patent documents 18 and 19); and a super-water-absorbent resincomposition comprising a super-water-absorbent resin and a fine powderof an aggregate of a hydro-oxide which contains two kinds of metals M1and M2 at least partially having an -M1-O-M2- bond (patent document 20).

As to these publicly known methods (patent documents 15 to 20), thegel-blocking can be prevented, but there have occurred problems suchthat the durability of the performance to diffuse liquids in diapers,particularly, the Saline Flow Conductivity (hereinafter abbreviated toSFC), is low. Or, even if the performance to diffuse the liquids isenough, there is not taken into consideration various performancedeteriorations due to such as mechanical impact or friction which thewater-absorbent resin undergoes when it is produced or used to produceabsorbent articles, and therefore no sufficient performance can bemaintained in the actual production. For example, as the case may be,even if improvement effects are seen in laboratories, those effects arenot seen or are deteriorated when the production is carried out with aproduction machine involving the step in which physical energy worksagainst the powder such as stirring or pneumatic transportation.

There is known a water-absorbing agent comprising 100 weight parts ofwater-absorbent resin particles and 1 to 30 weight parts of aheat-fusible resin powder having a melting point in the range of 50 to160° C. (patent document 21).

In this art (patent document 21), there is disclosed a method in whichthe water-absorbent resin particles and the heat-fusible resin powderhaving a melting point in the range of 50 to 160° C. are heat-treatedafter or during their mixing, whereby the heat-fusible resin powder isfixed to the water-absorbent resin particles. Such a heat-fusible resinpowder is used for the purpose of enhancing the fixability to fiberssuch as pulp, in other words, as a binder for the fibers and thewater-absorbent resin particles. However, such a heat-fusible resinpowder enhances the fixability of the water-absorbent resin particles tothe fibers, but has no interactions with a carboxyl group. Therefore, ithas been impossible to obtain the effect of enhancing the liquidpermeability and liquid diffusibility of the water-absorbent resin. Inaddition, in the case where the heat-fusible resin powder has stronghydrophobicity, it may cause such as deterioration of the capillarysuction force of the resultant water-absorbent resin composition.Therefore, the resultant water-absorbent resin composition has notnecessarily been a water-absorbent resin composition having sufficientperformances.

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DISCLOSURE OF THE INVENTION Objects of the Invention

An object of the present invention is to provide a water-absorbent resincomposition and its production process, wherein the water-absorbentresin composition causes little gel-blocking and is excellent in theliquid permeability and liquid diffusibility and is high also in theabsorption performances (e.g. absorption capacity without load,absorption capacity under load, capillary absorption capacity) andfurther is strong also against the physical damage such as during theactual production or practical use.

Also, another object of the present invention is to provide awater-absorbent resin composition and its production process, whereinthe water-absorbent resin composition has the following furtheradvantages, in addition to the above, of involving little segregation ofthe metal compound (e.g. metal salt), and being excellent also in thehandling property during the moisture absorption, and further having adust prevention effect.

SUMMARY OF THE INVENTION

The present inventors diligently studied to solve the aforementionedproblems. As a result, they have found out that: if there is constructeda water-absorbent resin composition which comprises water-absorbentresin particles and water-soluble polyvalent metal salt particles,wherein the water-absorbent resin particles are obtained by polymerizinga monomer including acrylic acid and/or its salt, and wherein thewater-absorbent resin particles are surface-crosslinked ones, andwherein the water-absorbent resin composition has a mass-averageparticle diameter of 100 to 600 μm, then the above problems cansuccessfully be solved, because the resultant water-absorbent resincomposition causes little gel-blocking and is excellent in the liquidpermeability and liquid diffusibility and is high also in the absorptionperformances (e.g. absorption capacity without load, absorption capacityunder load, capillary absorption capacity) and further is strong alsoagainst the physical damage such as during the actual production orpractical use. At the same time, the present inventors have found outalso that: such a water-absorbent resin composition is a water-absorbentresin composition which displays a high saline flow conductivity (SFC)and is excellent in the retention ratio of the saline flow conductivity(SFC) after a paint shaker test and in the retention ratio of the salineflow conductivity (SFC) after a long-term liquid absorption.

In addition, the present inventors have further found out that: if, in aprocess for production of a water-absorbent resin composition in which apolyvalent metal is fixed to surfaces of water-absorbent resinparticles, there are involved a step in which a binder (e.g. water) isbeforehand added to the water-absorbent resin particles to thus put themin a state where the binder is permeated across surfaces of thewater-absorbent resin particles and a step in which they are thereaftermixed with water-soluble polyvalent metal salt particles, then the aboveproblems can successfully be solved, because: the permeation of thepolyvalent metal into the water-absorbent resin particles caneffectively be prevented, and further, the polyvalent metal is fixed allover the surfaces of the water-absorbent resin particles uniformly andmoderately (i.e. in a state where the fixation is incomplete, but is notso weak as to enable free migration), and consequently, the gel-blockingcan sufficiently be prevented, and therefore excellent liquidpermeability and liquid diffusibility can be displayed and alsoexcellent absorption performances can be displayed, and further, theresultant water-absorbent resin composition comes into a state which isstrong also against the physical damage such as during the actualproduction or practical use.

Moreover, the present inventors have further found out that: if at leasta part of a metal compound is fused to surfaces of water-absorbent resinparticles in a water-absorbent resin composition comprising thewater-absorbent resin particles and the metal compound wherein thewater-absorbent resin particles are obtained by polymerizing a monomerincluding acrylic acid and/or its salt, then surprisingly thegel-blocking can sufficiently be prevented, and therefore excellentliquid permeability and liquid diffusibility can be displayed and alsoexcellent absorption performances can be displayed, and further, theresultant water-absorbent resin composition comes into a state which isstrong also against the physical damage such as during the actualproduction or practical use, and besides, this composition involveslittle segregation of the metal compound (e.g. metal salt), and isexcellent also in the handling property during the moisture absorption,and further has a dust prevention effect.

That is to say, a first water-absorbent resin composition according tothe present invention (which may hereinafter be referred to aswater-absorbent resin composition (1)) is a water-absorbent resincomposition comprising water-absorbent resin particles obtained bypolymerizing a monomer including acrylic acid and/or its salt,

with the composition having a mass-average particle diameter of 100 to600 μm and comprising water-soluble polyvalent metal salt particles andthe water-absorbent resin particles that have been surface-crosslinked.

As to the first water-absorbent resin composition according to thepresent invention, it is favorable that at least a part of thewater-absorbent resin particles are agglomerates.

As to the first water-absorbent resin composition according to thepresent invention, it is favorable that the water-soluble polyvalentmetal salt particles are particles of an aluminum salt having water ofcrystallization.

As to the first water-absorbent resin composition according to thepresent invention, it is favorable that the water-absorbent resinparticles are those which have been surface-crosslinked with apolyhydric alcohol.

Also, the first water-absorbent resin composition according to thepresent invention is a water-absorbent resin composition comprisingwater-absorbent resin particles and water-soluble polyvalent metal saltparticles, wherein the water-absorbent resin particles are obtained bypolymerizing a monomer including acrylic acid and/or its salt,

with the composition of which the saline flow conductivity is at least50 (×10⁻⁷·cm³·s·g⁻¹) and of which the retention ratio of the saline flowconductivity is not less than 40%.

As to the first water-absorbent resin composition according to thepresent invention, favorably, its retention ratio of the saline flowconductivity after a paint shaker test is not less than 70%.

A first process according to the present invention for production of awater-absorbent resin composition (which may hereinafter be referred toas production process (1)) is characterized by comprising the steps of:

adding a binder to water-absorbent resin particles obtained bypolymerizing a monomer including acrylic acid and/or its salt; and then

mixing the binder and the water-absorbent resin particles withwater-soluble polyvalent metal salt particles.

As to the first process according to the present invention forproduction of a water-absorbent resin composition, it is favorable thatthe water-absorbent resin particles are surface-crosslinked ones.

As to the first process according to the present invention forproduction of a water-absorbent resin composition, it is favorable thatthe binder contains a surface-crosslinking agent.

As to the first process according to the present invention forproduction of a water-absorbent resin composition, it is favorable thatthe binder includes water and/or a polyhydric alcohol.

As to the first process according to the present invention forproduction of a water-absorbent resin composition, it is favorable that,when the binder is added to the water-absorbent resin particles, thetemperature of the water-absorbent resin particles is in the range of 40to 100° C.

A second water-absorbent resin composition according to the presentinvention (which may hereinafter be referred to as water-absorbent resincomposition (2)) comprises water-absorbent resin particles and a metalcompound, wherein the water-absorbent resin particles are obtained bypolymerizing a monomer including acrylic acid and/or its salt, andwherein:

the metal compound is one or not fewer than two members selected fromamong alkaline metal salts and polyvalent metal salts (except polyvalentmetal salts of organic acids having not fewer than 7 carbon atoms permolecule); and

at least a part of the metal compound is fused to surfaces of thewater-absorbent resin particles.

As to the second water-absorbent resin composition according to thepresent invention, it is favorable that the water-absorbent resinparticles are materials having been surface-crosslinked with a compoundhaving at least two functional groups which make a dehydration reactionor transesterification reaction with a carboxyl group.

As to the second water-absorbent resin composition according to thepresent invention, it is favorable that at least a part of the metalcompound is fused in the form of coating at least a part of surfaces ofthe water-absorbent resin particles in a layered state

As to the second water-absorbent resin composition according to thepresent invention, it is favorable that the metal compound has a meltingpoint of not higher than 250° C.

As to the second water-absorbent resin composition according to thepresent invention, it is favorable that the metal compound is awater-soluble polyvalent metal salt.

As to the second water-absorbent resin composition according to thepresent invention, it is favorable that the metal compound is awater-soluble polyvalent metal salt having water of hydration andcontaining aluminum.

As to the second water-absorbent resin composition according to thepresent invention, favorably, it displays an absorption capacity of notless than 20 g/g under load.

As to the second water-absorbent resin composition according to thepresent invention, favorably, it displays a saline flow conductivity ofnot less than 30 (×10⁻⁷·cm³·s·g⁻¹) for a 0.69 mass % physiologicalsaline solution.

A second process according to the present invention for production of awater-absorbent resin composition (which may hereinafter be referred toas production process (2)) is a process for production of awater-absorbent resin composition which includes water-absorbent resinparticles and a metal compound, wherein the water-absorbent resinparticles are obtained by polymerizing a monomer including acrylic acidand/or its salt, and wherein:

the metal compound is one or not fewer than two members selected fromamong alkaline metal salts and polyvalent metal salts (except polyvalentmetal salts of organic acids having not fewer than 7 carbon atoms permolecule);

with the process comprising the steps of:

heating the water-absorbent resin particles and/or the metal compound toa temperature of not lower than the melting point of the metal compound;and

thereby fusing at least a part of the metal compound to surfaces of thewater-absorbent resin particles.

As to the second process according to the present invention forproduction of a water-absorbent resin composition, it is favorable thatthe fusion is carried out under stirring of the water-absorbent resinparticles and/or the metal compound.

As to the second process according to the present invention forproduction of a water-absorbent resin composition, it is favorable thatthe fusion is carried out after a surface-crosslinking treatment of thewater-absorbent resin particles.

As to the second process according to the present invention forproduction of a water-absorbent resin composition, it is favorable thatthe metal compound has a melting point of not higher than 250° C.

As to the second process according to the present invention forproduction of a water-absorbent resin composition, it is favorable thatthe metal compound is a water-soluble polyvalent metal salt having waterof hydration and containing aluminum.

Effects of the Invention

The present invention can provide a water-absorbent resin compositionand its production process, wherein the water-absorbent resincomposition causes little gel-blocking and is excellent in the liquidpermeability and liquid diffusibility and is high also in the absorptionperformances (e.g. absorption capacity without load, absorption capacityunder load, capillary absorption capacity) and further is strong alsoagainst the physical damage such as during the actual production orpractical use. Also, the present invention can further provide awater-absorbent resin composition and its production process, whereinthe water-absorbent resin composition has the following further effects,in addition to the above, of involving little segregation of the metalcompound (e.g. metal salt), and being excellent also in the handlingproperty during the moisture absorption, and further having a dustprevention effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a measurement apparatus as usedfor measuring the AAP (method A).

FIG. 2 is a schematic sectional view of a measurement apparatus as usedfor measuring the AAP (method B).

FIG. 3 is a schematic sectional view of a measurement apparatus as usedfor measuring the SFC.

FIG. 4 is a schematic sectional view of a measurement apparatus as usedfor measuring the CSF.

FIG. 5 is a view (FIG. 5-(a)) obtained by taking an electronphotomicrograph of the water-absorbent resin composition (22) and, as tothis photomicrograph, a view (FIG. 5-(b)) obtained by taking an X-rayimage photomicrograph of the sulfur element by an SEM-EDS.

FIG. 6 is a view (FIG. 6-(a)) obtained by taking an electronphotomicrograph of the water-absorbent resin composition (22) and, as tothis photomicrograph, a view (FIG. 6-(b)) obtained by taking an X-rayimage photomicrograph of the sulfur element by an SEM-EDS.

EXPLANATION OF THE SYMBOLS

 1: Porous glass plate  2: Glass filter  3: Conduit  4: Liquid storagecontainer  5: Supporting ring  6: 0.90 mass % physiological salinesolution  7: Balance  8: Stand  9: Specimen to be measured (e.g.water-absorbent resin particles or water-absorbent resin composition)10: Load (0.41 kPa (0.06 psi)) 11: Air-intake pipe 12: Conduit 13: Glassfilter 14: 0.90 mass % physiological saline solution 15: Liquid storagecontainer 16: Balance 17: Filter paper 18: Metal gauze 19: Plasticcylinder 20: Load (2.07 kPa (0.3 psi)) 21: Load (4.83 kPa (0.7 psi)) 31:Tank 32: Glass tube 33: 0.69 mass % aqueous sodium chloride solution 34:L-tube having cock 35: Cock 40: Receptacle 41: Cell 42: Stainless metalgauze 43: Stainless metal gauze 44: Swollen gel 45: Glass filter 46:Piston 47: Holes in piston 48: Collecting receptacle 49: Balance 100: Plastic supporting cylinder 101:  Stainless metal gauze of 400 meshes102:  Swollen gel 103:  Piston 104:  Load (weight) 105:  Petri dish106:  Glass filter plate 107:  Filter paper 108:  0.90 mass %physiological saline solution

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention is described in detail. Incidentally,the water-absorbent resin composition according to the present inventionis a composition comprising a water-absorbent resin (water-absorbentresin particles) as the main component, and is a particulate compositioncomprising the water-absorbent resin in an amount of favorably 80 to 100mass % (or weight %: in the present invention, the weight and the masshave the same meaning, and their uses herein are unified into the mass),more favorably 90 to 100 mass %, and is used favorably for sanitarymaterials (e.g. disposable diapers, sanitary napkins, incontinent pads,and medical pads).

[Water-Absorbent Resin Particles]:

The water-absorbent resin particles, as used in the present invention,are particles of a water-insoluble, water-swellable, andhydrogel-formable polymer (which may hereinafter be referred to aswater-absorbent resin) obtainable by a process including the step ofpolymerizing a hydrophilic monomer, and has an absorption capacity of atleast not less than 10 times for a physiological saline solution, and isthe shape of spherical or irregular particles. Incidentally, in thepresent invention, the water-absorbent resin particles may be referredto simply as water-absorbent resin.

Specific examples of the water-insoluble, water-swellable, andhydrogel-formable polymer include: partially-neutralized and crosslinkedpolymers of poly(acrylic acids) (e.g. U.S. Pat. No. 4,625,001, U.S. Pat.No. 4,654,039, U.S. Pat. No. 5,250,640, U.S. Pat. No. 5,275,773, EP0456136); crosslinked and partially-neutralized graft polymers ofstarch-acrylic acid (U.S. Pat. No. 4,076,663); copolymers ofisobutylene-maleic acid (U.S. Pat. No. 4,389,513); saponified copolymersof vinyl acetate-acrylic acid (U.S. Pat. No. 4,124,748); hydrolyzed(co)polymers of acrylamide (U.S. Pat. No. 3,959,569); and hydrolyzedpolymers of acrylonitrile (U.S. Pat. No. 3,935,099). The water-absorbentresin, as used in the present invention, is favorably a water-absorbentresin including a crosslinked poly(acrylic acid) (salt) polymer obtainedby a process including the step of polymerizing a monomer includingacrylic acid and/or its salt. The crosslinked poly(acrylic acid) (salt)polymer in the present invention is a crosslinked polymer obtained by aprocess including the step of polymerizing a monomer including acrylicacid and/or its salt in an amount of not smaller than 50 mol %,favorably not smaller than 70 mol %, more favorably not smaller than 90mol %. In addition, it is favorable that 50 to 90 mol %, preferably 60to 80 mol %, of acid groups in the polymer are neutralized. As examplesof the salt, there can be cited such as: alkaline metal (e.g. sodium,potassium, lithium) salts, ammonium salts, and amine salts. Theneutralization of the water-absorbent resin for forming the salt may becarried out in a monomer state before the polymerization, or may becarried out in a polymer state on the way of or after thepolymerization, or may be carried out both in these states.

The crosslinked poly(acrylic acid) (salt) polymer, which is awater-absorbent resin as favorably used in the present invention, may bea copolymer obtained by copolymerizing another monomer jointly with themonomer used as the main component (acrylic acid and/or its salt), ifnecessary. Specific examples of the above other monomer include: anionicunsaturated monomers (e.g. methacrylic acid, maleic acid, vinylsulfonicacid, styrenesulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonicacid, 2-(meth)acryloylethanesulfonic acid,2-(meth)acryloylpropanesulfonic acid) and their salts;nonionic-hydrophilic-group-containing unsaturated monomers (e.g.acrylamide, methacrylamide, N-ethyl(meth)acrylamide,N-n-propyl(meth)acrylamide, N-isopropyl(meth)acrylamide,N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate,2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol(meth)acrylate, polyethylene glycol mono(meth)acrylate, vinylpyridine,N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine,N-vinylacetamide); and cationic unsaturated monomers (e.g.N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl (meth)acrylate,N,N-dimethylaminopropyl(meth)acrylamide, and their quaternary salts).The amount of these monomers used as monomers other than acrylic acidand/or its salt is favorably in the range of 0 to 30 mol %, morefavorably 0 to 10 mol %, of the entire monomers.

The water-absorbent resin, as used in the present invention, is acrosslinked polymer having a internal crosslinked structure.

As to methods for introducing the internal crosslinked structure intothe water-absorbent resin as used in the present invention, examplesthereof include: a method in which the introduction is carried out byself-crosslinking without any crosslinking agent; and a method in whichthe introduction is carried out by copolymerization or reaction with aninternal-crosslinking agent having at least two polymerizableunsaturated groups and/or at least two reactive groups per molecule. Afavorable example is the method in which the introduction is carried outby copolymerization or reaction with the internal-crosslinking agent.

Specific examples of these internal-crosslinking agents include:N,N′-methylenebis(meth)acrylamide, (poly)ethylene glycoldi(meth)acrylate, (poly)propylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, trimethylolpropanedi(meth)acrylate, glycerol tri(meth)acrylate, glycerol acrylatemethacrylate, ethylene-oxide-modified trimethylolpropanetri(meth)acrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallylisocyanurate, triallyl phosphate, triallylamine,poly(meth)allyloxyalkanes, (poly)ethylene glycol diglycidyl ether,glycerol diglycidyl ether, ethylene glycol, polyethylene glycol,propylene glycol, glycerol, pentaerythritol, ethylenediamine,polyethylenimine, and glycidyl (meth)acrylate. Theseinternal-crosslinking agents may be used either alone respectively or incombinations with each other. Above all, from the viewpoint of such aswater absorption properties of the obtained water-absorbent resin, it isfavorable that a compound having at least two polymerizable unsaturatedgroups is essentially used as the internal-crosslinking agent. Theamount of the above internal-crosslinking agent as used is favorably inthe range of 0.005 to 3 mol %, more favorably 0.01 to 1.5 mol %,relative to the entire monomers.

When the polymerization is carried out, there can be added such as:hydrophilic polymers (e.g. starch, cellulose, starch derivatives,cellulose derivatives, polyvinyl alcohol, poly(acrylic acid) (salts),and crosslinked poly(acrylic acid) (salts)); and chain transfer agentssuch as hypophosphorous acid (salts).

When the above monomer including acrylic acid and/or its salt as themajor component is polymerized to obtain the water-absorbent resin usedin the present invention, then bulk polymerization, reversed-phasesuspension polymerization, or precipitation polymerization may becarried out, but, from the viewpoint of the performance or the easinessin controlling the polymerization, it is favorable to carry out aqueoussolution polymerization in which the monomer is used in the form of anaqueous solution. Such polymerization methods are disclosed in such asU.S. Pat. No. 4,625,001, U.S. Pat. No. 4,769,427, U.S. Pat. No.4,873,299, U.S. Pat. No. 4,093,776, U.S. Pat. No. 4,367,323, U.S. Pat.No. 4,446,261, U.S. Pat. No. 4,683,274, U.S. Pat. No. 4,690,996, U.S.Pat. No. 4,721,647, U.S. Pat. No. 4,738,867, U.S. Pat. No. 4,748,076,and EP 1178059.

When the polymerization is carried out, there may, for example, be usedthe following: radical polymerization initiators such as potassiumpersulfate, ammonium persulfate, sodium persulfate, t-butylhydroperoxide, hydrogen peroxide, and 2,2′-azobis(2-amidinopropane)dihydrochloride; and active energy rays such as ultraviolet rays andelectron beams. In addition, in the case where the radicalpolymerization initiators are used, they may be used jointly withreducing agents such as sodium sulfite, sodium hydrogensulfite, ferroussulfate, and L-ascorbic acid to carry out redox polymerization. Theamount of these polymerization initiators as used is favorably in therange of 0.001 to 2 mol %, more favorably 0.01 to 0.5 mol %, relative tothe entire monomers.

The shape of the water-absorbent resin, obtained by the abovepolymerization, is generally such as irregularly pulverized shape,spherical shape, fibrous shape, bar shape, approximately sphericalshape, or flat shape. However, the water-absorbent resin as used in thepresent invention is, desirably, particulate. If a water-absorbent resinof the irregularly pulverized shape as obtained by pulverization afterdrying is used, there are advantages in that the effects of the presentinvention are more enhanced.

The water-absorbent resin, as used in the present invention, isfavorably that of which the surfaces and their neighborhood have furtherbeen crosslinked with a surface-crosslinking agent.

Examples of the surface-crosslinking agent usable for thesurface-crosslinking treatment include: organic surface-crosslinkingagents which have at least two functional groups reactable with afunctional group (particularly, a carboxyl group) of the water-absorbentresin; and polyvalent metal compounds. Examples thereof include:polyhydric alcohol compounds (e.g. ethylene glycol, diethylene glycol,propylene glycol, triethylene glycol, tetraethylene glycol, polyethyleneglycol, 1,3-propanediol, dipropylene glycol,2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol,polyglycerol, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol,1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine,polyoxypropylene, oxyethylene-oxypropylene block copolymers,pentaerythritol, and sorbitol); epoxy compounds (e.g. ethylene glycoldiglycidyl ether, polyethylene glycol diglycidyl ether, glycerolpolyglycidyl ether, diglycerol polyglycidyl ether, polyglycerolpolyglycidyl ether, propylene glycol diglycidyl ether, polypropyleneglycol diglycidyl ether, and glycidol); polyamine compounds (e.g.ethylenediamine, diethylenetriamine, triethylenetetramine,tetraethylenepentamine, pentaethylenehexamine, and polyethylenimine) andtheir inorganic or organic salts (e.g. azetidinium salts);polyisocyanate compounds (e.g. 2,4-tolylene diisocyanate andhexamethylene diisocyanate); polyoxazoline compounds (e.g.1,2-ethylenebisoxazoline); carbonic acid derivatives (e.g. urea,thiourea, guanidine, dicyandiamide, 2-oxazolidinone); alkylene carbonatecompounds (e.g. 1,3-dioxolan-2-one, 4-methyl-1,3-dioxolan-2-one,4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one,4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one,1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one,4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxopan-2-one); haloepoxycompounds (e.g. epichlorohydrin, epibromohydrin, and(α-methylepichlorohydrin) and their polyamine-added products (e.g.Kymene (registered trademark) produced by Hercules); silane couplingagents (e.g. γ-glycidoxypropyltrimethoxysilane andγ-aminopropyltriethoxysilane); oxetane compounds (e.g.3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol,3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol,3-ethyl-3-oxetaneethanol, 3-butyl-3-oxetaneethanol,3-chloromethyl-3-methyloxetane, 3-chloromethyl-3-ethyloxetane, andpolyoxetane compounds); and polyvalent metallic compounds (e.g.hydroxides and chlorides of such as zinc, calcium, magnesium, aluminum,iron and zirconium). These surface-crosslinking agents may be usedeither alone respectively or in combinations with each other. Above all,the polyhydric alcohols are favorable, because they are high in safetyand can enhance the hydrophilicity of water-absorbent resin particlesurfaces. In addition, the use of the polyhydric alcohols enhance theaffinity of water-absorbent resin particle surfaces to the polyvalentmetal particles, so that interactions between the polyhydric alcoholresidue and the polyvalent metal surface enables more uniform existenceof the polyvalent metal particles on surfaces of the water-absorbentresin particles.

The amount of the surface-crosslinking agent, as used, is favorably inthe range of 0.001 to 5 mass parts, per 100 mass parts of the solidcontent of the water-absorbent resin.

When the surface-crosslinking agent and the water-absorbent resin aremixed together, water may be used. The amount of water, as used, isfavorably larger than 0.5 but not larger than 10 mass parts, morefavorably in the range of 1 to 5 mass parts, per 100 mass parts of thesolid content of the water-absorbent resin.

When the surface-crosslinking agent and/or its aqueous solution ismixed, a hydrophilic organic solvent and/or a third substance may beused as a mixing assistant.

In the case where the hydrophilic organic solvent is used, its examplesinclude: lower alcohols (e.g. methyl alcohol, ethyl alcohol, n-propylalcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, andt-butyl alcohol); ketones (e.g. acetone); ethers (e.g. dioxane,tetrahydrofuran, and methoxy(poly)ethylene glycol); amides (e.g.ε-caprolactam and N,N-dimethylformamide); sulfoxides (e.g. dimethylsulfoxide); and polyhydric alcohols (e.g. ethylene glycol, diethyleneglycol, propylene glycol, triethylene glycol, tetraethylene glycol,polyethylene glycol, 1,3-propanediol, dipropylene glycol,2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol,polyglycerol, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol,1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine,polyoxypropylene, oxyethylene-oxypropylene block copolymers,pentaerythritol, and sorbitol). Though depending on such as kind,particle diameters, and water content of the water-absorbent resin, theamount of the hydrophilic organic solvent as used is favorably notlarger than 10 mass parts, more favorably in the range of 0.1 to 5 massparts, per 100 mass parts of the solid content of the water-absorbentresin. In addition, as the third substance, there may be caused tocoexist those which are disclosed in EP 0668080, such as inorganicacids, organic acids, and polyamino acids. These mixing assistants mayact as surface-crosslinking agents, but are favorably those which do notgive a surface-crosslinked water-absorbent resin having low waterabsorption performance. Particularly, volatile alcohols having boilingpoints of lower than 150° C. are desirable in that they volatilizeduring the surface-crosslinking treatment and thus their residues do notremain.

When the water-absorbent resin and the surface-crosslinking agent aremixed together, there may be caused to coexist a noncrosslinkablewater-soluble inorganic base (favorably: alkaline metal salts, ammoniumsalts, alkaline metal hydroxides, and ammonia or its hydroxide) and/oran irreducible alkaline-metal-salt pH buffer (favorably such ashydrogencarbonates, dihydrogenphosphates, and hydrogenphosphates) forthe purpose of more uniformly mixing the water-absorbent resin and thesurface-crosslinking agent together. The amount of these materials, asused, depends upon such as type or particle diameters of thewater-absorbent resin, but is favorably in the range of 0.005 to 10 massparts, more favorably 0.05 to 5 mass parts, per 100 mass parts of thesolid content of the water-absorbent resin.

Although not especially limited, the method for mixing thewater-absorbent resin and the surface-crosslinking agent together can beexemplified by such as: a method in which the water-absorbent resin isimmersed into the hydrophilic organic solvent and then mixed with thesurface-crosslinking agent that is, if necessary, dissolved in waterand/or a hydrophilic organic solvent; and a method in which thesurface-crosslinking agent that is dissolved in water and/or thehydrophilic organic solvent is spraywise or dropwise added directly tothe water-absorbent resin to mix them together.

After the mixing of the water-absorbent resin and thesurface-crosslinking agent, usually, a heating treatment is carried outto conduct the crosslinking reaction. Though depending on thesurface-crosslinking agent as used, the temperature of the above heatingtreatment is favorably in the range of 40 to 250° C., more favorably 150to 250° C. In the case where the treatment temperature is lower than 40°C., the absorption properties such as absorption capacity under a loadare sometimes not sufficiently improved. In the case where the treatmenttemperature is higher than 250° C., the deterioration of thewater-absorbent resin is sometimes caused, so that the performance islowered, therefore caution is needed. The duration of the heatingtreatment is favorably in the range of 1 minute to 2 hours, morefavorably 5 minutes to 1 hour.

There is no especial limitation on the particle diameters and particlediameter distribution of the water-absorbent resin as used in thepresent invention. However, if there is used a water-absorbent resinhaving comparatively small particle diameters and a particle diameterdistribution such that the content of components having small particlediameters is high, then there are advantages in that the waterabsorption performances such as water absorption rate and capillaryabsorption capacity are greatly enhanced.

As to the water-absorbent resin as used in the present invention, it isfavorable for enhancing the performances such as water absorption rateand capillary absorption capacity that the mass-average particlediameter is not larger than 500 μm, more favorably not larger than 400μm. In addition, the ratio of particles having particle diameters ofsmaller than 300 μm in the water-absorbent resin is favorably not lessthan 10 mass %, more favorably not less than 30 mass %, still morefavorably not less than 50 mass %, relative to the water-absorbentresin. A water-absorbent resin having such particle diameters either isthat which is obtained by pulverizing a water-absorbent resin obtainedby the aqueous solution polymerization, or can favorably be obtained bysieving the pulverized water-absorbent resin to thus regulate itsparticle diameters. In addition, it is also possible to use awater-absorbent resin obtained by a process including the steps of:agglomerating a fine powder of a water-absorbent resin having particlediameters of not larger than 300 μm; and then regulating the particlediameters of the agglomerated water-absorbent resin. Furthermore, it isalso possible to use a water-absorbent resin obtained by a process inwhich irregularly pulverized particles which are primary particlesobtained by pulverization are partially mixed with the agglomeratedmaterial of the fine powder. In the case of having been partially mixedwith the agglomerated material of the water-absorbent resin, there canbe obtained a water-absorbent resin composition according to the presentinvention which is still more excellent in the absorption propertiessuch as water absorption rate and capillary absorption capacity. Theamount of the agglomerated material of the fine powder, which is mixed,is favorably not smaller than 5 mass %, more favorably not smaller than10 mass %, still more favorably not smaller than 15 mass %.

As methods for preparing the agglomerated material of the fine powder,it is possible to use publicly known arts to regenerate a fine powder.Examples of such usable arts include methods in which: warm water and afine powder of a water-absorbent resin are mixed together and then dried(U.S. Pat. No. 6,228,930); a fine powder of a water-absorbent resin ismixed with an aqueous monomer solution, and then the resultant mixtureis polymerized (U.S. Pat. No. 5,264,495); water is added to a finepowder of a water-absorbent resin, and then the resultant mixture isagglomerated under not less than a specific face pressure (EP 0844270);a fine powder of a water-absorbent resin is sufficiently wetted to thusform an amorphous gel, and then this gel is dried and pulverized (U.S.Pat. No. 4,950,692); and a fine powder of a water-absorbent resin and apolymer gel are mixed together (U.S. Pat. No. 5,478,879). However, thereis favorably used the aforementioned method in which warm water and afine powder of a water-absorbent resin are mixed together and thendried. Incidentally, the particle diameter is indicated by the sievemesh opening size of the classification.

[Water-Soluble Polyvalent Metal Salt Particles]:

It has already been commonly known that a certain kind of inorganiccompound particles prevent the gel-blocking to thus provide awater-absorbent resin with high distributing and diffusing abilities.However, the present inventors studied about a water-absorbent resincomposition which displays high performance even in the case where, asis aforementioned, a water-absorbent resin has been damaged by such aspneumatic transportation which is usually included in processes forproduction of water-absorbent resins. As a result, the present inventorshave found out that: unexpectedly, in the case where the water-solublepolyvalent metal salt particles are used as the inorganic compoundparticles, the effect of enhancing the liquid permeation rate under loadis great and further, only in the case where the water-solublepolyvalent metal salt is not added to a water-absorbent resin in a stateof an aqueous solution, but the water-soluble polyvalent metal salt isadded to water-absorbent resin particles in a state of particles, thereis obtained a water-absorbent resin composition which is excellent bothin a performance of retaining the liquid permeation rate under load fora long time and in the physical damage resistance. In addition, thepresent inventors have found out also that: in the case where thewater-soluble polyvalent metal salt particles are hydrous salt crystals,particularly the effects are great. As to the reason why thewater-absorbent resin composition obtained by the dry mixing displayssuch effects, it is inferred (from great deterioration of the effects ofthe present invention as to a composition being in a state where thewater-soluble polyvalent metal salt particles are dissolved to thus bein a non-particulate state by adding the water-soluble polyvalent metalsalt to a water-absorbent resin in the form of an aqueous solution oradding water to a dry wise mixture of the water-absorbent resinparticles and the water-soluble polyvalent metal salt particles) that:if the water-soluble polyvalent metal salt is intermingled with thewater-absorbent resin particles in the form of particles, then, when thephysical damage such as impact is done to the water-absorbent resincomposition, the water-soluble polyvalent metal salt particles absorbthe impact energy to thus reduce the damage to the water-absorbentresin. Hereupon, the impact energy can be considered as being consumedby pulverization of the water-soluble polyvalent metal salt particlesand by uniformization due to rearrangement of the water-solublepolyvalent metal salt particles. Therefore, it can be considered asdesirable that the water-soluble polyvalent metal salt particles are ina state of the dry wise mixture which can move with some degree offreedom rather than are entirely fixed on surfaces of thewater-absorbent resin.

In addition to the above, the water-soluble polyvalent metal salt has anaction of hydrophilizing the surfaces of the water-absorbent resinparticles and, when the water-absorbent resin composition absorbs anaqueous liquid, the water-soluble polyvalent metal salt particlesdissolve to thus make actions of ion-crosslinking the surfaces of thewater-absorbent resin and keeping the spaces between water-absorbentresins wide. These actions have the effect of enhancing the liquidpermeation rate under load. Hereupon, this effect is greater when thepolyvalent metal exists in the periphery of and/or near the surfaces ofthe water-absorbent resin particles than when the polyvalent metalexists inside the water-absorbent resin particles. As to thewater-absorbent resin composition as produced by adding thewater-soluble polyvalent metal salt to a water-absorbent resin in theform of an aqueous solution or adding water to a dry wise mixture of thewater-absorbent resin particles and the water-soluble polyvalent metalsalt particles, much of the polyvalent metal salt has already permeatedthe inside of the water-absorbent resin, and therefore the effects ofthe polyvalent metal salt upon water-absorbent resin surfaces are lowduring the absorption of an aqueous liquid such as urine. As a result,the above water-absorbent resin composition is low in performance,particularly, in the liquid permeation rate under load, and itsdurability is also bad. In addition, because the permeated polyvalentmetal salt reacts with the carboxyl group to thus form a crosslinkedstructure, the liquid permeability is deteriorated due to the damagedone by the process. In comparison, because the water-soluble polyvalentmetal salt usable in the present invention is mixed with thewater-absorbent resin particles in the form of particles, thewater-soluble polyvalent metal salt does not dissolve to act on thewater-absorbent resin surfaces until the water-absorbent resincomposition absorbs urine or an aqueous liquid. This action can moreefficiently make the effects of the polyvalent metal salt upon thewater-absorbent resin surfaces endure for a long time. In addition, thewater-absorbent resin composition according to the present invention, inwhich the water-soluble polyvalent metal salt particles exist onsurfaces of the water-absorbent resin particles, is excellent in theliquid permeability still after having been damaged by the process,because the water-soluble polyvalent metal salt particles exist onsurfaces of the water-absorbent resin particles still after thecomposition has been damaged by the process. That it to say, thewater-absorbent resin composition according to the present invention canbe said to have a structure which effectively exercises the effects ofthe polyvalent metal without deteriorating the liquid absorptionperformance of the water-absorbent resin itself.

The water-soluble polyvalent metal salt particles, usable in the presentinvention, are particles of a salt of a metal having a valence of notless than 2 and are powdery. Taking it into consideration that thewater-absorbent resin composition according to the present invention isutilized for absorbent structures for sanitary materials such asdiapers, then it is favorable to select a water-soluble polyvalent metalsalt which does not color the water-absorbent resin composition andwhich is low poisonous to human bodies.

For the purpose of more efficiently making the effects of the polyvalentmetal salt endure for a long time during the liquid absorption, there isfavorably selected and used the polyvalent metal salt which can dissolvein a concentration of not less than 5 mass %, more favorably not lessthan 10 mass %, still more favorably not less than 20 mass %, in purewater of normal temperature.

Examples of the water-soluble polyvalent metal salt particles, usable inthe present invention, include aluminum chloride, poly(aluminumchloride), aluminum sulfate, aluminum nitrate, potassium aluminumbis(sulfate), sodium aluminum bis(sulfate), calcium chloride, calciumnitrate, magnesium chloride, magnesium sulfate, magnesium nitrate, zincchloride, zinc sulfate, zinc nitrate, zirconium chloride, zirconiumsulfate, and zirconium nitrate. In addition, also from the viewpoint ofsolubility into a liquid being absorbed such as urine, it is favorableto use these salts having water of crystallization. Particularlyfavorable are the aluminum compounds, above all, aluminum sulfate.Powders of hydrous crystals such as aluminum sulfate octadecahydrate andaluminum sulfate tetradeca- to octadecahydrates can most favorably beused.

From the viewpoint of mixability, it is favorable that the particlediameters of the water-soluble polyvalent metal salt particles, usablein the present invention, are smaller than those of the water-absorbentresin. The mass-average particle diameter is favorably not larger than500 μm, more favorably not larger than 400 μm. From the viewpoint ofperformance, the water-soluble polyvalent metal salt particles includeparticles having particle diameters of not larger than 150 μm in anamount of more favorably not smaller than 20 mass %, most favorably notsmaller than 30 mass %, relative to the water-soluble polyvalent metalsalt particles.

As the behavior and state of the water-soluble polyvalent metal saltparticles usable in the present invention, it is favorable from theviewpoint of damage mitigation that the particles are such asagglomerates. The bulk density is favorably not less than 0.5 g/cm³,more favorably not less than 0.7 g/cm³.

[Water-Absorbent Resin Composition (1)]:

The water-absorbent resin composition (1) according to the presentinvention is a water-absorbent resin composition comprisingwater-absorbent resin particles obtained by polymerizing a monomerincluding acrylic acid and/or its salt,

with the composition having a mass-average particle diameter of 100 to600 μm and comprising water-soluble polyvalent metal salt particles andthe water-absorbent resin particles that have been surface-crosslinked.

Also, the water-absorbent resin composition (1) according to the presentinvention is a water-absorbent resin composition comprisingwater-absorbent resin particles and water-soluble polyvalent metal saltparticles, wherein the water-absorbent resin particles are obtained bypolymerizing a monomer including acrylic acid and/or its salt,

with the composition of which the saline flow conductivity (SFC) is atleast 50 (×10⁻⁷·cm³·s·g⁻¹) and of which the retention ratio of thesaline flow conductivity (SFC) is not less than 40%.

The water-absorbent resin composition (1) according to the presentinvention comprises the water-absorbent resin particles as the maincomponent and further comprises the water-soluble polyvalent metal saltparticles, and is usually particulate and can be used favorably as anabsorbent material for sanitary materials for absorption of urine,menstrual blood, sweat, and other body fluids.

Because the water-soluble polyvalent metal salt particles in thewater-absorbent resin composition (1) according to the present inventionare merely mixed with the water-absorbent resin particles in the form ofparticles, the water-soluble polyvalent metal salt particles exist onsurfaces of the water-absorbent resin particles while still keepingtheir particulate shape, or exist in the periphery of thewater-absorbent resin particles, for example, between particles of thewater-absorbent resin, while still keeping their particulate shape. Afavorable mode is a state where the water-soluble polyvalent metal saltparticles substantially coexist uniformly with the water-absorbent resinparticles. If the water-soluble polyvalent metal salt particles canmaintain the particle shape in order to form such a state, then the modemay be that the water-soluble polyvalent metal salt particles are madeto adhere weakly to the water-absorbent resin particles with such as abinder. It may be possible to observe these states from photographstaken with such as electron microscopes. However, they can be confirmedby dispersing and stirring the water-absorbent resin composition into anappropriate organic solvent or an appropriate gas and then separatingthe water-absorbent resin particles and the water-soluble polyvalentmetal salt particles from each other by utilizing the difference betweentheir specific gravities.

The water-absorbent resin composition (1) according to the presentinvention, favorably, includes the water-absorbent resin particles andthe water-soluble polyvalent metal salt particles. At least a part ofthe aforementioned water-soluble polyvalent metal salt particles adhereweakly to water-absorbent resin particle surfaces by the binder.Therefore, the permeation of the polyvalent metal into thewater-absorbent resin particles is effectively prevented, and further,the polyvalent metal is fixed all over the surfaces of thewater-absorbent resin particles uniformly and moderately (i.e. in astate where the fixation is incomplete, but is not so weak as to enablefree migration). Consequently, the gel-blocking can sufficiently beprevented, and therefore excellent liquid permeability and liquiddiffusibility can be displayed and also excellent absorptionperformances can be displayed. Furthermore, the water-absorbent resincomposition comes into a state which is strong also against the physicaldamage such as during the actual production or practical use. Thesestates can also be observed from photographs taken with such as electronmicroscopes.

The water-absorbent resin particles, which are included in thewater-absorbent resin composition (1) according to the presentinvention, are, favorably, surface-crosslink-treated ones.

The water-absorbent resin composition (1), according to the presentinvention, is in the form of particles having a mass-average particlediameter in the range of favorably 100 to 600 μm, more favorably 200 to500 μm. In the case where the mass-average particle diameter is smallerthan 100 μm, then, even if the water-soluble polyvalent metal saltparticles are added, there is a possibility that it may be difficult toobtain the effects of the present invention. In the case where themass-average particle diameter is larger than 600 μm, then there is apossibility that the water-soluble polyvalent metal salt particles maysegregate to deteriorate the uniform mixability. In addition, the ratioof particles having particle diameters of smaller than 300 μm in thewater-absorbent resin composition (1) is favorably not less than 10 mass%, more favorably not less than 30 mass %, still more favorably not lessthan 50 mass %, relative to the water-absorbent resin composition (1).

The water-absorbent resin composition (1), according to the presentinvention, favorably displays an absorption capacity without load (CRC)of not less than 20 (g/g), more favorably not less than 22 (g/g), stillmore favorably not less than 24 (g/g), yet still more favorably not lessthan 25 (g/g), particularly favorably not less than 27 (g/g). In thecase where the absorption capacity without load (CRC) is less than 20(g/g), the absorption efficiency is bad on an occasion of the use forsanitary materials such as diapers.

The water-absorbent resin composition (1), according to the presentinvention, favorably displays an absorption capacity under load (AAP) ofnot less than 16 (g/g), more favorably not less than 18 (g/g), stillmore favorably not less than 20 (g/g), yet still more favorably not lessthan 22 (g/g), particularly favorably not less than 24 (g/g), under aload of 0.7 psi. In the case where the absorption capacity under load(AAP) is less than 16 (g/g), the absorption efficiency is bad on anoccasion of the use for sanitary materials such as diapers.

The water-absorbent resin composition (1), according to the presentinvention, favorably displays a saline flow conductivity (SFC) of notless than 50 (×10⁻⁷·cm³·s·g⁻¹), more favorably not less than 100(×10⁻⁷·cm³·s·g⁻¹), still more favorably not less than 120(×10⁻⁷·cm³·s·g⁻¹), particularly favorably not less than 150(×10⁻⁷·cm³·s·g⁻¹). The saline flow conductivity (SFC) depends on thecontent of the water-absorbent resin composition in the sanitarymaterial. The higher content needs the higher saline flow conductivity(SFC).

As to the water-absorbent resin composition (1) according to the presentinvention, it is desirable that the deterioration of the absorptioncapacity under load (AAP) of this water-absorbent resin composition, ascompared with an absorption capacity under load (AAP) (under the sameload) of the water-absorbent resin particles to which the water-solublepolyvalent metal salt particles have not yet been added, is small. Thewater-absorbent resin composition favorably maintains an absorptioncapacity under load of not less than 0.85 time, more favorably not lessthan 0.90 time, still more favorably not less than 0.95 time, incomparison with the absorption capacity under load (AAP) of thewater-absorbent resin particles.

The water-absorbent resin composition (1), according to the presentinvention, displays an effect such that the deterioration of absorptionperformances is small even if it undergoes various physical energy(damage) during the production or practical use. That is to say, thewater-absorbent resin composition (1), according to the presentinvention, is a water-absorbent resin composition which has thefollowing absorption performances after physical energy has workedagainst the composition.

The water-absorbent resin composition (1), according to the presentinvention, favorably displays an absorption capacity without load afterthe paint shaker test (shaking at 800 cycles/min for 30 minutes) (CRCafter PS) of not less than 20 (g/g), more favorably not less than 22(g/g), still more favorably not less than 24 (g/g), yet still morefavorably not less than 25 (g/g), particularly favorably not less than27 (g/g). In the case where the absorption capacity without load afterthe paint shaker test (CRC after PS) is less than 20 (g/g), theabsorption efficiency is bad on an occasion of the use for sanitarymaterials such as diapers.

The water-absorbent resin composition (1), according to the presentinvention, favorably displays a saline flow conductivity after the paintshaker test (SFC after PS) of not less than 50 (×10⁻⁷·cm³·s·g⁻¹), morefavorably not less than 100 (×10⁻⁷·cm³·s·g⁻), still more favorably notless than 120 (×10⁻⁷·cm³·s·g⁻¹), particularly favorably not less than150 (×10⁻⁷·cm³·s·g⁻¹). The saline flow conductivity after the paintshaker test (SFC after PS) depends on the content of the water-absorbentresin composition in the sanitary material. The higher content needs thehigher saline flow conductivity (SFC).

As to the water-absorbent resin composition (1) according to the presentinvention, the ratio of a saline flow conductivity (SFC) after the paintshaker test to an SFC before the PS test, namely, the retention ratio ofthe saline flow conductivity after the paint shaker test (retentionratio of SFC after PS), is favorably not less than 70%, more favorablynot less than 80%, still more favorably not less than 90%, particularlyfavorably not less than 100%.

The water-absorbent resin composition (1), according to the presentinvention, can maintain a high saline flow conductivity (SFC) even whenused in sanitary materials for a long time.

As to the water-absorbent resin composition (1) according to the presentinvention, the ratio of a saline flow conductivity (SFC) after aswelling time of 120 minutes to a saline flow conductivity (SFC) after aswelling time of 60 minutes, namely, the retention ratio of the salineflow conductivity (retention ratio of SFC), is favorably not less than40%, more favorably not less than 50%, still more favorably not lessthan 60%. As to conventional water-absorbent resins (or water-absorbentresin compositions) to which metal particles have been added, if theyare measured for a swelling duration of more than 60 minutes in the testfor the saline flow conductivity (SFC), then a rapid fall of the liquidpermeation rate is seen.

The water-absorbent resin composition (1), according to the presentinvention, further has a feature of generating little dust. As to thewater-absorbent resin composition (1) according to the presentinvention, the dust generation degree is favorably not more than 0.25(mg/m³), more favorably not more than 0.23 (mg/m³), still more favorablynot more than 0.20 (mg/m³), yet still more favorably not more than 0.17(mg/m³), particularly favorably not more than 0.15 (mg/m³).

The water-absorbent resin composition (1) according to the presentinvention is excellent in the wettability to aqueous liquids.Particularly above all, a water-absorbent resin composition includingwater-absorbent resin particles which have been surface-crosslinked withthe polyhydric alcohol is excellent in the wettability and contributesto the enhancement of the absorption performances. The wettability ofthe water-absorbent resin composition to aqueous liquids can beevaluated by measuring the contact angle. It is not easy to preciselymeasure the contact angle of a liquid with a liquid-absorbent powderlike the water-absorbent resin composition. However, the apparentcontact angle can be measured by the below-mentioned method. Thewater-absorbent resin composition according to the present inventiondisplays a contact angle of favorably not more than 45 degrees, morefavorably not more than 30 degrees, particularly favorably not more than20 degrees.

The water-absorbent resin composition (1) according to the presentinvention may possess such functions as given or enhanced by causingthis composition to, besides the water-absorbent resin particles and thewater-soluble polyvalent metal salt particles, further contain additivessuch as: water-insoluble finely-particulate inorganic powders (e.g.silicon dioxide, titanium dioxide, aluminum oxide, magnesium oxide, zincoxide, talc, calcium phosphate, barium phosphate, silicic acid or itssalts, clay, diatomite, zeolite, bentonite, kaolin, hydrotalcite, andsalts (e.g. activated clay)); deodorants, perfumes, antibacterialagents, cationic polymer compounds (e.g. polyamines), foaming agents,pigments, dyes, manures, oxidizing agents, and reducing agents. Theratio of the additives as used is less than 10 mass %, favorably lessthan 5 mass %, more favorably less than 1 mass %, relative to the totalof the water-absorbent resin particles and the water-soluble polyvalentmetal salt particles.

[Process (1) for Production of Water-Absorbent Resin Composition]:

A favorable process for production of the composition (1) according tothe present invention is a dry mixing process. The dry mixing process isa process in which the water-absorbent resin particles (favorably,surface-crosslinked ones) and the water-soluble polyvalent metal saltparticles are mixed together in a state where the water-solublepolyvalent metal salt particles substantially keep their dry state.Hereupon, the water-soluble polyvalent metal salt particles are mixedunder conditions where they can exist as independent particles.

When the water-absorbent resin composition (1) according to the presentinvention is produced or preserved, it must be avoided adding or mixingwater in such an amount that the water-soluble polyvalent metal saltparticles may dissolve, or putting the composition under high humidity.If the water-soluble polyvalent metal salt particles contact with thewater-absorbent resin in a state dissolved in such as water, then thewater-soluble polyvalent metal salt falls into a state coated towater-absorbent resin particle surfaces or permeated inside the resin tothus exist as particles no longer, so that the effects of the presentinvention are not sufficiently displayed. For example, inJP-A-523289/2001 (Kohyo) (WO 98/48857), there is disclosed a process forpreparation of a super-water-absorbent polymer which process ischaracterized by including the steps of mixing a super-water-absorbentpolymer with a polyvalent metal salt and then bringing the resultantmixture into close contact with a binder. Water or a water-solubleliquid is stated therein as the aforementioned binder. However, thepresent invention entirely differs from conventional processes in that,when the water-absorbent resin composition (1) according to the presentinvention is produced, an aqueous liquid (e.g. water, water-solubleliquid) does not need to be added after the mixing of thewater-absorbent resin particles and the water-soluble polyvalent metalsalt particles. Therefore, the water-soluble polyvalent metal saltparticles which are contained in the water-absorbent resin composition(1) according to the present invention are not dissolved in a liquid,but exist substantially as dry particles along with the water-absorbentresin. It is an important method according to the present invention thatthe water-soluble polyvalent metal salt particles are caused to exist ina state of dry particles. Thereby there can be obtained thewater-absorbent resin composition (1) which is excellent in waterabsorption performances such as after-damage liquid permeability.

The specific mixing method is free of especial limitation if it is a drymixing method. For example, publicly known methods for addition andmixing of powders are used to carry out the addition in a lump ordivisionally or continuously. The addition and mixing of thewater-soluble polyvalent metal salt particles may be carried out whilethe water-absorbent resin particles are stirred. Or the stirringoperation may be carried out after the addition of the water-solublepolyvalent metal salt particles. Usable as stirring apparatuses ormixing apparatuses are such as paddle blenders, ribbon mixers, rotaryblenders, jar tumblers, plowshare mixers, cylinder type mixers,V-character-shaped mixers, ribbon type mixers, screw type mixers,twin-arm mixers, pulverizing type kneaders, channel type mixers, andplow type mixers.

The ratio between the water-soluble polyvalent metal salt particles andthe water-absorbent resin particles as used is favorably in the range of0.01 to 5 mass parts, more favorably 0.1 to 2 mass parts, per 100 massparts of the solid content of the water-absorbent resin particles. Inthe case where the water-soluble polyvalent metal salt particles areadded too much, the performance deterioration of the water-absorbentresin is brought about. In the case where the amount of thewater-soluble polyvalent metal salt particles is too small, thewater-soluble polyvalent metal salt particles do not take effect.

A more favorable mode of the dry mixing processes, which are favorableprocesses for production of the composition (1) according to the presentinvention, is a mode comprising the steps of:

adding a binder to water-absorbent resin particles obtained bypolymerizing a monomer including acrylic acid and/or its salt; and then

mixing the binder and the water-absorbent resin particles withwater-soluble polyvalent metal salt particles.

Its feature is that: the binder is beforehand added to thewater-absorbent resin particles to thus put them in a state where thebinder is permeated across surfaces of the water-absorbent resinparticles, and then they are mixed with the water-soluble polyvalentmetal salt particles.

As to the modes as reported in JP-A-523287/2001 (Kohyo),JP-A-124879/1997 (Kokai), JP-A-270741/1988 (Kokai), and JP-A-538275/2002(Kohyo) aforementioned as background arts, namely, as to the modes thata metal salt is formed into its aqueous solution and then added to awater-absorbent resin, there have been problems such that: the metalunfavorably goes so far as permeating into particles of thewater-absorbent resin, so that there occur the following: thedeterioration of the absorption capacity without load by influence ofthe metal inside the water-absorbent resin; and the deterioration of theabsorption capacity under load, the gel-blocking, and the deteriorationsof the liquid permeability and liquid diffusibility, due to insufficientpresence of the metal on surfaces of the water-absorbent resin.

In addition, as to the modes as reported in JP-A-257235/1986 (Kokai) andJP-A-523289/2001 (Kohyo) aforementioned as background arts, namely, asto the modes that a water-absorbent resin is dry-blended with a metalsalt and then water is added to them, there have been problems suchthat: the dissolved metal salt causes binding between particles to thuseasily form a strong agglomerate and, in the case where this agglomerateis crushed by physical damage such as during the actual production orpractical use, the absorption capacity under load is deteriorated. Inaddition, there have also been problems such that: the dissolved metalsalt unfavorably goes so far as permeating into particles of thewater-absorbent resin.

On the other hand, as to the above more favorable mode, because thewater-soluble polyvalent metal salt particles being in a state of apowder is mixed with the water-absorbent resin particles in a statewhere the binder is permeated across surfaces of the water-absorbentresin particles, the permeation of the polyvalent metal into thewater-absorbent resin particles can effectively be prevented, andfurther, the polyvalent metal is fixed all over the surfaces of thewater-absorbent resin particles uniformly and moderately (i.e. in astate where the fixation is incomplete, but is not so weak as to enablefree migration). Thereby consequently, the gel-blocking can sufficientlybe prevented, and therefore excellent liquid permeability and liquiddiffusibility can be displayed and also excellent absorptionperformances can be displayed. Furthermore, the resultantwater-absorbent resin composition comes into a state which is strongalso against the physical damage such as during the actual production orpractical use.

The binder usable in the present invention has a role as a binder forfixing the polyvalent metal to surfaces of the water-absorbent resinparticles, and is beforehand added to the water-absorbent resinparticles before the water-absorbent resin particles is mixed with thewater-soluble polyvalent metal salt particles.

The binder usable in the present invention is free of especiallimitation if it includes a material which can play the above role.However, for example, there can be cited those which include such aswater, polyhydric alcohols, water-soluble polymers, thermoplasticresins, pressure-sensitive adhesives, and adhesives. Favorable are thosewhich include water and/or polyhydric alcohols.

The binder usable in the present invention may contain theaforementioned surface-crosslinking agent. Particularly, as is mentionedbelow, if the binder is made to contain the surface-crosslinking agentin the case where not yet surface-crosslinked water-absorbent resinparticles are used as the water-absorbent resin particles, then itbecomes possible to carry out the surface-crosslinking treatment in theprocess for production of the water-absorbent resin composition (1).

The amount of the binder usable in the present invention is favorably inthe range of 0.1 to 10 mass %, more favorably 0.1 to 5 mass %, stillmore favorably 0.2 to 3 mass %, relative to the solid content of thewater-absorbent resin particles. In the case where the amount of thebinder is smaller than 0.1 mass %, there is a possibility that themoderate fixation of the polyvalent metal cannot be realized. In thecase where the amount of the binder is larger than 10 mass %, there is apossibility that the properties of the resultant water-absorbent resincomposition (1) may be deteriorated.

The method for adding the binder to the water-absorbent resin particlesis not especially limited. However, there is preferred a method whichenables uniform addition of the binder to the water-absorbent resinparticles to mix them together. For instance, it can be exemplified bysuch as a method in which the binder is spraywise or dropwise addeddirectly to the water-absorbent resin particles to mix them together.Examples of apparatuses for the mixing include cylinder type mixers,V-character-shaped mixers, ribbon type mixers, screw type mixers,twin-arm mixers, pulverizing type kneaders, channel type mixers, andplow type mixers.

When the binder is added to the water-absorbent resin particles, it isfavorable to beforehand adjust the temperature of the water-absorbentresin particles in the range of 40 to 100° C., more favorably 50 to 90°C., still more favorably 60 to 80° C. In the case of deviating from theabove temperature ranges, it is difficult that the binder is uniformlyadded to the water-absorbent resin particles to mix them together.

In the above more favorable mode, either surface-crosslinkedwater-absorbent resin particles or not yet surface-crosslinkedwater-absorbent resin particles may be used as the water-absorbent resinparticles. If the binder is made to contain the surface-crosslinkingagent in the case where the not yet surface-crosslinked water-absorbentresin particles are used as the water-absorbent resin particles, then itbecomes possible to carry out the surface-crosslinking treatment in theprocess for production of the water-absorbent resin composition (1).

In the above more favorable mode, the water-absorbent resin particles towhich the binder has been added is mixed with the water-solublepolyvalent metal salt particles.

The method for mixing the water-absorbent resin particles with thewater-soluble polyvalent metal salt particles is the same as theaforementioned.

In the process (1) according to the present invention for production ofa water-absorbent resin composition, the stirring operation is notnecessarily needed when the water-soluble polyvalent metal saltparticles are added to the water-absorbent resin particles. After thewater-soluble polyvalent metal salt particles have been added to thewater-absorbent resin particles, the step in which physical energy suchas impact works against water-absorbent resins (which step is usuallyincluded in processes for production of the water-absorbent resins) maybe utilized still in a state of non-uniform mixing, thus carrying outthe mixing.

A favorable mode for carrying out the present invention is a processwhich utilizes energy that works against the water-absorbent resin when,as often adopted in processes for production of water-absorbent resins,the water-absorbent resin particles (powder) are pneumaticallytransported. That is to say, if the water-soluble polyvalent metal saltparticles are added in a step as carried out before the step ofpneumatically transporting the water-absorbent resin powder and ifthereafter the pneumatic transportation is carried out, then thewater-absorbent resin composition which is excellent in such asabsorption performances can be obtained without needing to be processedwith any special mixer or pulverizer.

The pneumatic transportation of the water-absorbent resin particles(powder) in the process for production of the water-absorbent resin isusually carried out over a transportation distance of 10 to 200 m and ata transportation speed of 0.1 to 15 m/second. By passing through thepneumatic transportation step, the water-absorbent resin particles andthe water-soluble polyvalent metal salt particles are pulverized andmixed together due to contact and collision between particles or due tocollision of particles with walls of the transportation course, so thatthe water-absorbent resin composition according to the present inventioncan be obtained.

The energy which the water-absorbent resin particles and thewater-soluble polyvalent metal salt particles undergo during thepneumatic transportation corresponds to energy which they undergo whenbeing shaken with a paint shaker in a state where they are placed in acontainer together with glass beads. Therefore, the energy to beactually applied can be reproduced with the paint shaker in a laboratory(paint shaker test, which may hereinafter be abbreviated as PS). Thepaint shaker test (PS) is carried out as follows: a glass container of 6cm in diameter and 11 cm in height is charged with 10 g of glass beadsof 6 mm in diameter and 30 g of the water-absorbent resin composition,and then attached to a paint shaker (product No. 488, produced by ToyoSeiki Seisakusho K.K.), and then shaken at 800 cycles/min (CPM) for 30minutes. The details of the apparatus are disclosed in JP-A-235378/1997(Kokai). The energy which is applied to the water-absorbent resinparticles and the water-soluble polyvalent metal salt particles isenergy corresponding to the range of 5 to 60 minutes, preferably 5 to 30minutes, as the duration of the aforementioned shaking with the paintshaker, and almost conforms to the aforementioned energy as appliedduring the pneumatic transportation. It is also possible to make designof the pneumatic transportation step, such as pneumatic transportationdistance and pneumatic transportation speed, by changing the shakingduration of the PS in the above range to thus find out the optimum pointfor performances of the water-absorbent resin composition. In addition,it is also possible to determine the selection or amount of thewater-soluble polyvalent metal salt particles being used, by carryingout a test of mixing the water-absorbent resin particles and thewater-soluble polyvalent metal salt particles together for the shakingduration of the PS corresponding to energy of the already designedpneumatic transportation step. If such force is applied, a part of thewater-soluble polyvalent metal salt particles as used in the presentinvention become extremely fine particles to come into a state wherethey adhere uniformly to water-absorbent resin particle surfaces.

That is to say, in a favorable process for production of thewater-absorbent resin composition (1) according to the presentinvention, it is favorable that, for the water-absorbent resinparticles, there are used the water-soluble polyvalent metal saltparticles which are in itself so brittle as to be pulverized due to theabove physical energy, such as being in the form of: a powder ofcrystals; or an agglomerate or agglomerated material of fine particles;so that a mixture in which fine particles of the water-solublepolyvalent metal salt particles adhere uniformly to the water-absorbentresin particles (powder) can be obtained even if no specialpulverization apparatus is used.

[Water-Absorbent Resin Composition (2)]:

The water-absorbent resin composition (2) according to the presentinvention comprises water-absorbent resin particles and a metalcompound, wherein the water-absorbent resin particles are obtained bypolymerizing a monomer including acrylic acid and/or its salt, andwherein:

the metal compound is one or not fewer than two members selected fromamong alkaline metal salts and polyvalent metal salts (except polyvalentmetal salts of organic acids having not fewer than 7 carbon atoms permolecule); and

at least a part of the metal compound is fused to surfaces of thewater-absorbent resin particles.

As to the metal compound as used in the present invention, taking itinto consideration that the water-absorbent resin composition (2)according to the present invention is utilized as a water-absorbingagent for sanitary materials such as disposable diapers, then it isfavorable to select a metal compound which does not color thewater-absorbent resin composition (2) and which is low poisonous tohuman bodies.

The metal compound, as used in the present invention, is favorably oneor not fewer than two members selected from among alkaline metal saltsand polyvalent metal salts (except polyvalent metal salts of organicacids having not fewer than 7 carbon atoms per molecule). Because thepolyvalent metal salts of organic acids having not fewer than 7 carbonatoms per molecule have such high hydrophobicity as to deteriorate thecapillary suction force of the water-absorbent resin composition, suchpolyvalent metal salts are not used as the polyvalent metal salts in thepresent invention.

Favorable as the alkaline metal salts are salts of Li, Na, and K.

The polyvalent metal salts except the polyvalent metal salts of organicacids having not fewer than 7 carbon atoms per molecule are favorablythose which contain one or not fewer than two polyvalent metals selectedfrom among Be, Mg, Ca, Sc, Y, Ti, Zr, V, Nb, Cr, Mo, Mn, Fe, Ru, Co, Rh,Ni, Pd, Cu, Zn, Ga, Ge, and Al, and more favorably those which containone or not fewer than two polyvalent metals selected from among Ca, Mg,Fe, Ti, Zr, Zn, and Al, and most favorably those which contain Al. Inaddition, the aforementioned water-soluble polyvalent metal saltparticles may be used.

The metal compound, as used in the present invention, is favorably ametal compound having a melting point of not higher than 250° C. Inaddition, the melting point of the metal compound is favorably in therange of 30 to 250° C., more favorably 40 to 200° C., still morefavorably 50 to 150° C., most favorably 60 to 100° C. In the case wherethe melting point is higher than 250° C., there is a possibility that,when the metal compound is fused to the water-absorbent resin particles,damage may be done thereto, thus resulting in failure to obtain theobjective properties. In addition, in the case where the melting pointis lower than 30° C., there is a possibility that the metal compound mayunfavorably permeate the inside of the water-absorbent resin particles,thus resulting in failure to obtain the objective properties.

The metal compound, as used in the present invention, is favorablyhydrophilic and/or water-soluble. Therefore, the polyvalent metal saltsof organic acids having not fewer than 7 carbon atoms per molecule areunfavorable. In addition, in the case where the polyvalent metal saltsof organic acids having not fewer than 7 carbon atoms per molecule areused, their hydrophobicity is too high, and therefore there is apossibility that they may cause such as deterioration of the surfacetension of the water-absorbent resin particles, thus resulting infailure to obtain the objective properties. In the present invention,the water solubility refers to a compound of which not less than 1 g,favorably not less than 10 g, dissolves into 100 g of water at 25° C.

The metal compound, as used in the present invention, favorably haswater of hydration in its molecule. The metal compound having the waterof hydration is usually hydrophilic and easily takes effect when thewater-absorbent resin composition (2) according to the present inventionabsorbs water.

The metal compound, as used in the present invention, is favorably asolid, more favorably a powder, at normal temperature. In the case wherethe metal compound is a powder, then, the finer its particle diametersare, the more easily it fuses with the water-absorbent resin particles.Therefore, its mass-average particle diameter (D50) is favorably notlarger than 1,000 μm, more favorably not larger than 600 μm, still morefavorably not larger than 300 μm, most favorably not larger than 150 μm.

Specific examples of the metal compound, as used in the presentinvention, include one or not fewer than two members selected from amongalkaline metal salts and polyvalent metal salts having not more than 6carbon atoms which are recorded as of Sep. 10, 2003 in a GMELIN fileprovided by Gmelin Institute Varrentrappstr. and/or a BEILSTEIN fileprovided by Beilstein Chemiedaten und Software GmbH (each of them isusable as a search file of STN INTERNATIONAL, and their agency in Japanis the Scientific Information Society of Japan). As favorable ones amongthem, there can be cited those which have melting points of not higherthan 250° C. As more favorable ones among them, there can be cited oneor not fewer than two members selected from among inorganic acid saltsor salts of organic acids having not more than 6 carbon atoms permolecule, such as sulfates, nitrates, phosphates, halides, oxalates, andacetates. As still more favorable ones among them, there can be citedthose which have water of hydration. Being exemplified morespecifically, there are favorably used one or not fewer than two membersselected from among Al₂(SO₄)₃.14-18H₂O, KAl(SO₄)₂.12H₂O,(NH₄)Al(SO₄)₂.12H₂O, NaAl(SO₄)₂.12H₂O, AlCl₃.6H₂O, Na₂B₄O₇.5-10H₂O,FeSO₄.7H₂O, and K₂SO₄.Fe₂(SO₄)₃.24H₂O. The most favorable is one or notfewer than two members selected from among Al₂(SO₄)₃.14-18H₂O andKAl(SO₄)₂.12H₂O.

The water-absorbent resin composition (2) according to the presentinvention comprises the water-absorbent resin particles and the metalcompound, and is usually particulate and can be used favorably as anabsorbent material for sanitary materials for absorption of urine,menstrual blood, sweat, and other body fluids.

The water-absorbent resin composition (2), according to the presentinvention, is a water-absorbent resin composition which comprises thewater-absorbent resin particles and the metal compound, wherein thewater-absorbent resin particles are obtained by polymerizing a monomerincluding acrylic acid and/or its salt, and wherein: the metal compoundis one or not fewer than two members selected from among alkaline metalsalts and polyvalent metal salts having not more than 6 carbon atoms;and at least a part of the metal compound is fused to surfaces of thewater-absorbent resin particles.

In the present invention, the term “fusion” refers to a state where atleast a part of the metal compound is made to adhere strongly tosurfaces of the water-absorbent resin particles in a melted state.

In the present invention, favorably, the water-absorbent resin particlesare surface-crosslinked ones. In addition, more favorably, thewater-absorbent resin particles are materials having beensurface-crosslinked with a polyhydric alcohol. Even if a small amount ofsurface-treating agent remains on surfaces, the metal compound is notbound to surfaces of the water-absorbent resin particles merely bymixing the remaining-surface-treating-agent-containing water-absorbentresin particles and the metal compound together. Therefore, theremaining surface-treating agent is not usable as the binder.

The water-absorbent resin composition (2), according to the presentinvention, is favorably free from binder as used to bind thewater-absorbent resin particles and the metal compound to each other.The binder has a possibility of causing the deteriorations of thesurface tension, the capillary absorption capacity (CSF), and otherproperties. For obtaining the water-absorbent resin composition (2)according to the present invention, it is not necessary to bind thewater-absorbent resin particles and the metal compound to each otherwith the binder. The fusion of the metal compound to surfaces of thewater-absorbent resin particles can strongly bind them together. As thecase may be, the use of the binder rather hinders the fusion of themetal compound to the water-absorbent resin particles.

In the water-absorbent resin composition (2) according to the presentinvention, favorably, at least a part of the aforementioned metalcompound is fused in the form of coating a part of surfaces of thewater-absorbent resin particles in a layered state. The layered staterefers to a state where the aforementioned metal compound thinly coatssurfaces of the water-absorbent resin particles. The fusion in the formof the coating in the layered state prevents the metal compound frompermeating the inside of the water-absorbent resin particles and alsoincreases the surface area, and therefore makes it possible to easilyobtain the objective performances. In addition, a form such that thesurfaces of the water-absorbent resin particles are all coated with themetal compound has a possibility of bringing about deterioration ofproperties. Therefore, it is favorable that the metal compound forms adiscontinuous layer on surfaces of the water-absorbent resin particles.In addition, there is also preferred a form such that inter-particularbinding between the aforementioned water-absorbent resin particles andthe aforementioned metal compound is formed by the fusion. It may bepossible to observe these states from photographs taken with such aselectron microscopes.

The water-absorbent resin composition (2), according to the presentinvention, is in the form of particles having a mass-average particlediameter in the range of favorably 100 to 600 μm, more favorably 200 to500 μm. In the case where the mass-average particle diameter is smallerthan 100 μm, then there is a possibility that: the handling property maybe bad, and also much dust may be contained, and the liquid permeabilityand the liquid diffusibility may be bad. In the case where themass-average particle diameter is larger than 600 μm, then there is apossibility that damage nay be easily done, thus resulting indeterioration of properties.

As to the water-absorbent resin composition (2) according to the presentinvention, the logarithmic standard deviation (σζ) of the particlediameter distribution is favorably in the range of 0.25 to 0.45, morefavorably 0.27 to 0.47, still more favorably 0.30 to 0.40. Thelogarithmic standard deviation (σζ) of the particle diameterdistribution is a numerical value indicating the broadness of theparticle diameter distribution. The smaller this value is, the narrowerparticle diameter distribution it shows. That is to say, in the casewhere the logarithmic standard deviation (σζ) is more than 0.45, thereis a possibility that the width of the particle diameter distributionmay be too broad, thus resulting in bad handling property or in failureto obtain the objective properties. In the case where the logarithmicstandard deviation (σζ) is less than 0.25, there is a possibility thatthe productivity may greatly be deteriorated, thus resulting in failureto obtain the effects corresponding to the cost.

As to the water-absorbent resin composition (2) according to the presentinvention, because at least a part of the metal compound is fused tosurfaces of the water-absorbent resin particles, segregation of themetal compound occurs little. As an index for knowing the tendency forthis segregation to occur, there is a metal compound segregation index.Specifically, the metal compound segregation index of thewater-absorbent resin composition (2) according to the present inventionis favorably in the range of 0.0 to 2.0, more favorably 0.0 to 1.7, mostfavorably 0.0 to 1.5. The metal compound segregation index is determinedby the below-mentioned method.

As to the water-absorbent resin composition (2) according to the presentinvention, when at least a part of the metal compound fuses to surfacesof the water-absorbent resin particles, fine particles also fuse as thecase may be. Therefore, the dust prevention effect is high.

It has been found out by the present inventors that the water-absorbentresin composition (2) according to the present invention is,surprisingly, more excellent in the handling property during themoisture absorption when compared with other methods for adding themetal compound to the water-absorbent resin particles (e.g. dry mixing,aqueous solution addition, addition of a binder after dry mixing). Inthe case where the metal compound is fused to surfaces of thewater-absorbent resin particles, the metal compound coats a part ofsurfaces of the water-absorbent resin particles, whereby the bindingbetween the water-absorbent resin particles during the moistureabsorption is inhibited, so that there is provided an effect ofpreventing the agglomeration between particles. This effect works moreeffectively by the fusion. For example, in the case where the metalcompound is added in the form of an aqueous solution, the metal compoundunfavorably permeates the inside of the particles, and it is thereforedifficult to obtain the effect of preventing the agglomeration betweenparticles. In addition, in the case of the dry mixing, thewater-absorbent resin particles and the metal compound contact with eachother at points, thus still resulting in failure to obtain so mucheffect as that in the case of the fusion.

As to the water-absorbent resin composition (2) according to the presentinvention, the blocking ratio (BR) when having been put at 25° C. and arelative humidity of 70% for 1 hour is favorably not more than 20%, morefavorably not more than 10%, still more favorably not more than 5%.

The water-absorbent resin composition (2), according to the presentinvention, favorably displays an absorption capacity without load (CRC)in the range of 15 to 45 g/g, more favorably 20 to 37 g/g, still morefavorably 24 to 32 g/g. In the case where the absorption capacitywithout load is low, there is a possibility that the efficiency may bebad on an occasion of the use for sanitary materials such as diapers. Inthe case where the absorption capacity without load is too high, thereis a possibility that the performance deterioration may occur due tosuch as deterioration of gel strength.

The water-absorbent resin composition (2), according to the presentinvention, favorably displays an absorption capacity under load (AAP) ofnot less than 16 g/g, more favorably not less than 20 g/g, still morefavorably not less than 24 g/g. In addition, it is desirable that thedeterioration of the absorption capacity under load (AAP) of thewater-absorbent resin composition (2), as compared with an absorptioncapacity under load (AAP) of the water-absorbent resin particles towhich the metal compound has not yet been added, is small. Thewater-absorbent resin composition (2) favorably maintains an absorptioncapacity under load (AAP) of not less than 0.85 time, more favorably notless than 0.90 time, most favorably not less than 0.95 time, incomparison with the absorption capacity under load (AAP) of thewater-absorbent resin particles.

The water-absorbent resin composition (2), according to the presentinvention, favorably displays a saline flow conductivity (SFC) of notless than 30 (×10⁻⁷·cm³·s·g⁻¹), more favorably not less than 50(×10⁻⁷·cm³·s·g⁻¹), still more favorably not less than 80(×10⁻⁷·cm³·s·g⁻¹), most favorably not less than 100 (×10⁻⁷·cm³·s·g⁻¹).The saline flow conductivity (SFC) is a numerical value indicating theliquid permeability and the liquid diffusibility. The higher this valueis, the more excellent in the liquid permeability and the liquiddiffusibility the water-absorbent resin composition is. The saline flowconductivity (SFC) depends on the content of the water-absorbent resincomposition in the sanitary material. The higher content needs thehigher value of the saline flow conductivity (SFC).

The water-absorbent resin composition (2), according to the presentinvention, favorably displays a capillary absorption capacity (CSF) ofnot less than 15 g/g, more favorably not less than 18 g/g, mostfavorably not less than 20 g/g, at a height of 20 cm. The capillaryabsorption capacity (CSF) is a value indicating the strength of thecapillary suction force. The higher capillary absorption capacity (CSF)can more diffuse an absorbed liquid also in a height direction and istherefore more desirable.

As the water-absorbent resin composition (2) according to the presentinvention, there are used those of which the water-extractable componentcontent is favorably not higher than 20 mass %, more favorably nothigher than 15 mass %, most favorably not higher than 10 mass %. In thecase where the water-extractable component content is higher than 20mass % in the present invention, there is a possibility not only that noeffects of the present invention may be obtained, but also that theperformance may be deteriorated in the use for water-absorbentstructures for sanitary materials such as diapers. In addition, such awater-extractable component content is unfavorable also from theviewpoint of safety. As a cause of the performance deterioration, it canbe cited that, when the water-absorbent resin composition absorbs waterto swell, a high-molecular component elutes from the inside of thewater-absorbent resin to thereby hinder the liquid permeation. Theeluted high-molecular component can be considered to resist when aliquid flows across surfaces of water-absorbent resin particles. Inaddition, similarly, the elution of the high-molecular component has apossibility of increasing the viscosity of an absorbed solution to thusdeteriorate the capillary suction force. The water-extractable componentcontent of the water-absorbent resin composition is measured by thebelow-mentioned method.

Although not especially limited, the water content of thewater-absorbent resin composition (2) according to the present inventionis favorably in the range of 0 to 100 mass %, more favorably 0.01 to 40mass %, still more favorably 0.1 to 10 mass %.

Although not especially limited, the bulk density of the water-absorbentresin composition (2) according to the present invention is favorably inthe range of 0.40 to 0.90 g/ml, more favorably 0.50 to 0.80 g/ml (themethod for measuring the bulk density is specified in JIS K-3362). Inthe case of water-absorbent resin compositions which have a bulk densityof less than 0.40 g/ml or more than 0.90 g/ml, there is a possibilitythat they may be damaged easily by the process and may accordingly bedeteriorated in property.

The water-absorbent resin composition (2) according to the presentinvention may possess such functions as given or enhanced by causingthis composition to, besides the water-absorbent resin particles and themetal compound, further contain additives such as: water-insolublefinely-particulate inorganic powders (e.g. silicon dioxide, titaniumdioxide, aluminum oxide, magnesium oxide, zinc oxide, talc, boric acid,silicic acid, clay, diatomite, zeolite, bentonite, kaolin, hydrotalcite,and salts (e.g. activated clay)); deodorants, perfumes, antibacterialagents, cationic polymer compounds (e.g. polyamines), foaming agents,pigments, dyes, manures, oxidizing agents, and reducing agents. Theratio of the additives as used is favorably less than 10 mass %, morefavorably less than 5 mass %, still more favorably less than 1 mass %,relative to the mass of the water-absorbent resin composition.

In the case where the water-absorbent resin composition (2) according tothe present invention is used for the sanitary materials, if the metalcompound is fused to the surface-crosslinked water-absorbent resinparticles, then the wettability to aqueous liquids is good, and further,a liquid-absorbed gel little causes what is called gel-blocking, andspaces between gel particles are not clogged up due to close cohesion ofthe gel, either. Therefore, even in the case where the water-absorbentresin composition is used in a high concentration in absorbentstructures such as diapers, it is possible that, at the second time orthereafter, urine or body fluids diffuse into the absorbent structures,without losing a place to go on surfaces of the absorbent structures, sothat the urine or body fluids can be distributed to the insidewater-absorbent resin particles. Furthermore, a mixture of thewater-absorbent resin of the water-absorbent resin composition and anagglomerated material of this water-absorbent resin composition hasspaces of the appropriate size between particles and therefore combinesa property of sucking a liquid by the capillary force and therefore candiffuse an absorbed liquid into the entire absorbent structure also bythe capillary suction force.

[Process (2) for Production of Water-Absorbent Resin Composition]:

The water-absorbent resin particles and the metal compound, which areused for the production of the water-absorbent resin composition (2)according to the present invention, are as previously explained.

The process for production of the water-absorbent resin composition (2)according to the present invention is a process comprising the steps of:heating the water-absorbent resin particles and/or the metal compound toa temperature of not lower than the melting point of the metal compound;and thereby fusing at least a part of the metal compound to surfaces ofthe water-absorbent resin particles; wherein the water-absorbent resinparticles are obtained by polymerizing a monomer including acrylic acidand/or its salt.

In the present invention, the term “fusion” refers to a phenomenon suchthat at least a part of the metal compound melts by heat or becomessofter by heat than its solid state, thus adhering to another substance,and the term “fusion” is used in the same meaning as heat-fusion.However, there is also included a case where the metal compound is fusedby melting it or making it softer than its solid state, by means otherthan heat.

That is to say, when it comes to the above process, there is no especiallimitation. However, for example, there are the following processes (a)to (d):

(a) A process in which the water-absorbent resin particles, which havebeen heated to not lower than the melting point of the metal compound,and the metal compound are mixed together.

(b) A process in which the metal compound, which has been heated to notlower than the melting point of the metal compound, and thewater-absorbent resin particles are mixed together.

(c) A process in which the water-absorbent resin particles and the metalcompound are mixed together, and then the resultant mixture is heated tonot lower than the melting point of the metal compound.

(d) A process in which the water-absorbent resin particles, which havebeen heated to not lower than the melting point of the metal compound,and the metal compound, which has been heated to not lower than themelting point of the metal compound, are mixed together.

These processes are favorable, but other processes may be used.

In the present invention, examples of means of heating thewater-absorbent resin particles and/or the metal compound includeheaters, microwaves, ultrasonic waves, and far infrared rays.

In the present invention, favorably, the water-absorbent resin particlesare materials having been surface-crosslink-treated with a compoundhaving at least two functional groups which make a dehydration reactionor transesterification reaction with a carboxyl group.

In the present invention, the metal compound is used favorably in theform of not such as an aqueous dispersion or solution, but a powder.However, the metal compound may be used in a melted state without addingsuch as water thereto.

In the present invention, the metal compound is used favorably in anamount of 0.001 to 10 mass %, more favorably 0.01 to 5 mass %, mostfavorably 0.1 to 3 mass %, relative to the mass of the water-absorbentresin particles. In the case where the amount of the metal compoundbeing added is smaller than 0.001 mass %, it is difficult to obtain theeffects of the present invention. Also, in the case of the additionamount larger than 10 mass %, not only are there economicaldisadvantages, but also there is a possibility that the performancedeterioration of the water-absorbent resin composition may be broughtabout.

In the present invention, there is no especial limitation on the addingand mixing method. Publicly known methods for addition and mixing ofpowders may be used. In a favorable method, a predetermined amount ofmetal compound is added to the water-absorbent resin particles in a lumpor divisionally or continuously.

In the present invention, favorably, the mixing of the water-absorbentresin particles and the metal compound is carried out under stirring.Also favorably under stirring, at least a part of the metal compound isfused to surfaces of the water-absorbent resin particles. Usable asstirring apparatuses are such as paddle blenders, ribbon mixers, rotaryblenders, jar tumblers, plowshare mixers, and mortar mixers. Thesestirring apparatuses may be heatable apparatuses or may be apparatuseswhich cools the heated mixture.

In the present invention, when the water-absorbent resin particlesand/or the metal compound is heated, the water-absorbent resin particlesand/or the metal compound needs to be heated to not lower than themelting point of the metal compound. The temperature range to which theheating is carried out is favorably not higher than 250° C., morefavorably the range of 30 to 250° C., most favorably 50 to 200° C. Inaddition, a state where the metal compound does not entirely melt isfavorable. In the case where the metal compound has entirely melted,there is a possibility that the water-absorbent resin particles may becoated entirely with the metal compound, thus resulting in deteriorationof properties.

In the present invention, the stirring duration of the water-absorbentresin particles and the metal compound is not especially limited.However, it is favorably not more than 60 minutes, more favorably notmore than 30 minutes.

In the present invention, when at least a part of the metal compound isfused to surfaces of the water-absorbent resin particles, it isfavorable to give pressure to a mixture of the water-absorbent resinparticles and the metal compound. This pressurization promotes thefusion.

In the present invention, after at least a part of the metal compoundhas been fused to surfaces of the water-absorbent resin particles, it isfavorable to regulate the particle diameter distribution of theresultant water-absorbent resin composition. As means for regulating theparticle diameter distribution, it is enough to use publicly knownmeans. However, a disintegrator and/or a classifier is favorably used.

In the present invention, when the water-absorbent resin particles andthe metal compound are mixed together, addition of water and mixing ofthe metal compound in an aqueous solution state must be avoided. In thecase where the addition of water or the mixing of the metal compound inan aqueous solution state is made, the metal compound componentunfavorably permeates the inside of the water-absorbent resin particles,thus resulting in failure to sufficiently display the effects of thepresent invention. For example, in JP-A-523289/2001 (Kohyo) (WO98/48857), there is disclosed a process for preparation of asuper-water-absorbent polymer which process is characterized byincluding the steps of mixing a super-water-absorbent polymer with apolyvalent metal salt and then bringing the resultant mixture into closecontact with a binder. Water or a water-soluble liquid is stated thereinas the aforementioned binder. On the other hand, the present inventionentirely differs from conventional processes in that, when thewater-absorbent resin composition (2) according to the present inventionis produced, an aqueous liquid (e.g. water, water-soluble liquid) doesnot need to be added after the mixing of the water-absorbent resinparticles and the metal compound. Therefore, in the present invention,methods without using any binder are favorable. The use of the binderhas a possibility of deteriorating the surface tension, the CSF, andother properties. In addition, as an example of methods without usingthe water or the water-soluble liquid, there can be considered a methodin which the water-absorbent resin particles and the metal compound aremixed together in a dry manner. However, this method has a possibilityof causing such as segregation or increase of dust. In addition, thereare problems such that: even if the metal compound adheres to thewater-absorbent resin particles, its binding force is so weak that themetal compound is unfavorably easily released merely by application of alittle force. In the present invention, it is an important method tofuse at least a part of the metal compound to surfaces of thewater-absorbent resin particles. By this method, it is possible toprovide the water-absorbent resin composition which solves theaforementioned prior problems, and involves little segregation of theadditive, and is excellent in the liquid permeability and the liquiddiffusibility, and also, undergoes little deterioration of theabsorption capacity under load, and further, surprisingly, is excellentalso in the handling property during the moisture absorption, andfurther has a dust prevention effect.

[Water-Absorbent Structure]:

The water-absorbent resin composition (1) and/or (2), according to thepresent invention, can be combined with an appropriate material andthereby formed into the water-absorbent structure which is, for example,favorable as an absorbent layer for sanitary materials. Hereinafter, adescription is made about the water-absorbent structure in the presentinvention.

The water-absorbent structure in the present invention refers to amolded composition which comprises a water-absorbent resin compositionand another material and is used for sanitary materials (e.g. disposablediapers, sanitary napkins, incontinent pads, and medical pads) forabsorption of such as blood, body fluids, and urine. Examples of theabove other material include cellulose fibers. Specific examples of thecellulose fibers include: wood pulp fibers from wood, such as mechanicalpulp, chemical pulp, semichemical pulp, and dissolving pulp; andsynthetic cellulose fibers, such as rayon and acetate. Favorablecellulose fibers are the wood pulp fibers. These cellulose fibers maypartially contain synthetic fibers such as nylon and polyester. When thewater-absorbent resin composition (1) and/or (2) according to thepresent invention is used as a portion of the water-absorbent structure,the mass of the water-absorbent resin composition (1) and/or (2)according to the present invention as contained in the water-absorbentstructure is favorably in the range of not smaller than 20 mass %. Inthe case where the mass of the water-absorbent resin composition (1)and/or (2) according to the present invention as contained in thewater-absorbent structure is smaller than 20 mass %, there is apossibility that no sufficient effects can be obtained.

For the purpose of obtaining the water-absorbent structure from thewater-absorbent resin composition (1) and/or (2) according to thepresent invention and the cellulose fibers, for example, publicly knownmeans for obtaining water-absorbent structures can appropriate beselected from among such as: a method in which the water-absorbent resincomposition (1) and/or (2) is spread onto paper or mat made of such asthe cellulose fibers and is, if necessary, interposed therebetween; anda method in which the cellulose fibers and the water-absorbent resincomposition (1) and/or (2) are uniformly blended together. A favorablemethod is a method in which the water-absorbent resin composition (1)and/or (2) and the cellulose fibers are mixed together in a dry mannerand then compressed. This method can greatly prevent the water-absorbentresin composition (1) and/or (2) from falling off from the cellulosefibers. The compression is favorably carried out under heating, and itstemperature range is, for example, the range of 50 to 200° C. Inaddition, for the purpose of obtaining the water-absorbent structure,methods as disclosed in JP-A-509591/1997 (Kohyo) and JP-A-290000/1997(Kokai) are also favorably used.

In the case where used for water-absorbent structures, thewater-absorbent resin composition (1) and/or (2) according to thepresent invention is so excellent in the properties as to givewater-absorbent structures which are very excellent in that they quicklytake liquids in and further in that the amount of the liquids remainingon their surface layers is small.

Because the water-absorbent resin compositions (1) and (2) according tothe present invention have excellent water absorption properties, thesewater-absorbent resin compositions can be used as water-absorbing andwater-retaining agents for various purposes. For example, thesewater-absorbent resin compositions can be used for such as:water-absorbing and water-retaining agents for absorbent articles (e.g.disposable diapers, sanitary napkins, incontinent pads, and medicalpads); agricultural and horticultural water-retaining agents (e.g.substitutes for peat moss, soil-modifying-and-improving agents,water-retaining agents, and agents for duration of effects ofagricultural chemicals); water-retaining agents for buildings (e.g.dew-condensation-preventing agents for interior wall materials, cementadditives); release control agents; coldness-retaining agents;disposable portable body warmers; sludge-solidifying agents;freshness-retaining agents for foods; ion-exchange column materials;dehydrating agents for sludge or oil; desiccating agents; andhumidity-adjusting materials. In addition, the water-absorbent resincompositions (1) and (2) according to the present invention can be usedparticularly favorably for sanitary materials for absorption ofexcrement, urine, or blood, such as disposable diapers and sanitarynapkins.

In the case where the water-absorbent structure in the present inventionis used for sanitary materials (e.g. disposable diapers, sanitarynapkins, incontinent pads, and medical pads), this water-absorbentstructure is used favorably with a constitution including: (a) aliquid-permeable top sheet placed so as to be adjacent to a wearer'sbody; (b) a liquid-impermeable back sheet placed so as to be adjacent tothe wearer's clothes at a distance from the wearer's body; and (c) thewater-absorbent structure placed between the top sheet and the backsheet. The water-absorbent structure may be in more than one layer orused along with such as a pulp layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention is more specifically illustrated bythe following Examples of some preferred embodiments in comparison withComparative Examples not according to the present invention. However,the present invention is not limited to them. Hereinafter, forconvenience, the units “mass part(s)” and “liter(s)” may be referred tosimply as “part(s)” and “L” respectively. In addition, the unit “mass %”may be referred to as “wt %”.

The measurement and evaluation methods in the Examples and theComparative Examples are shown below.

Unless otherwise noted, the following measurement is stated as havingbeen carried out under conditions of a room temperature (25° C.) and ahumidity of 50 RH %.

Incidentally, in cases of water-absorbent resin compositions having beenused for end products such as sanitary materials, the water-absorbentresin compositions have already absorbed moisture. Therefore, themeasurement may be carried out after appropriately separating thewater-absorbent resin compositions from the end products and then dryingthe separated water-absorbent resin compositions under a reducedpressure at a low temperature (e.g. under not higher than 1 mmHg at 60°C. for 12 hours). In addition, all the water-absorbent resincompositions as used in the Examples and Comparative Examples of thepresent invention had water contents of not higher than 6 mass %.

<Absorption Capacity Without Load (CRC)>:

An amount of 0.20 g of water-absorbent resin or water-absorbent resincomposition was weighed out precisely to a level of 0.0001 g, and thenuniformly placed and sealed into a bag (85 mm×60 mm or 60 mm×60 mm) madeof nonwoven fabric (trade name: Heatron Paper, type: GSP-22, produced byNangoku Pulp Kogyo Co., Ltd.).

A container of 1 L was charged with 1 L of 0.9 mass % aqueous sodiumchloride solution (physiological saline solution), in which oneevaluation sample per one container was then immersed for 1 hour.Incidentally, because the present invention is an invention made bydirecting attention to effects of ion migration, more than one sampleper one container must not be immersed.

After 1 hour, the bag was pulled up and then drained of water bycentrifugal force of 250 G with a centrifugal separator (produced byKokusan Co., Ltd., centrifugal separator: model H-122) for 3 minutes,and then the mass W1 (g) of the bag was measured. In addition, the sameprocedure as the above was carried out without the water-absorbent resinor water-absorbent resin composition, and the resultant mass W0 (g) wasmeasured. Then, the absorption capacity (g/g) without load wascalculated from these W1 and W0 in accordance with the followingequation:CRC(g/g)=[(W1(g)−W0(g))/mass (g) of water-absorbent resin orwater-absorbent resin composition]−1

<Absorption Capacity Under Load (AAP)>:

The AAP was measured by the following method A or B. The AAP may bemeasured by either of these methods, and the measured value is littleinfluenced by the measurement method.

As to the AAP in the below-mentioned Referential Examples, Examples, andComparative Examples, the AAP as shown in Table 1 is AAP as measured bythe method A, and the other AAP is AAP as measured by the method B.

(Method A):

The absorption capacity under a load (AAP) was measured with anapparatus of FIG. 1.

There was prepared a load 21 as adjusted so as to give a pressure of4.83 kPa (0.7 psi). Onto a metal gauze 18 of 400 meshes (mesh openingsize of 38 μm) of a plastic cylinder 19 of 60 mm in diameter with themetal gauze stuck on its bottom, there was dispersed about 0.90 g (Wp2)of water-absorbent resin composition or water-absorbent resin, andfurther thereon the above load 21 (in the case of 0.7 psi) was put toprepare a liquid absorption instrument, which was then put on a filterpaper 17 on a glass filter 13 of FIG. 1. After 60 minutes, there wasmeasured a value (Wc) of the physiological saline solution (0.90 mass %aqueous NaCl solution) as absorbed by the water-absorbent resincomposition or water-absorbent resin. The absorption capacity under theload was determined using the following equation.AAP(g/g)=Wc/Wp2

(Method B):

The measurement was carried out with an apparatus as shown in FIG. 2.

A stainless metal gauze 101, which was a screen of 400 meshes (meshopening size: 38 μm), was attached by fusion to a bottom of a plasticsupporting cylinder 100 having an inner diameter of 60 mm. Then, underconditions of a room temperature (20 to 25° C.) and a humidity of 50 RH%, onto the above metal gauze, there was uniformly spread 0.90 g ofwater-absorbent resin or water-absorbent resin composition 102, andfurther thereon, there were mounted a piston 103 and a load 104 insequence, wherein the piston had an outer diameter of only a littlesmaller than 60 mm and made no gap with the inner wall surface of thesupporting cylinder, but was not hindered from moving up and down, andwherein the piston and the load were adjusted so that a load of 4.83 kPa(0.7 psi) could uniformly be applied to the water-absorbent resin orwater-absorbent resin composition. Then, the mass Wa (g) of theresultant one set of measurement apparatus was measured.

A glass filter plate 106 having a diameter of 90 mm (produced by SogoRikagaku Glass Seisakusho Co., Ltd., pore diameter: 100 to 120 μm) wasmounted inside a Petri dish 105 having a diameter of 150 mm, and then a0.90 mass % aqueous sodium chloride solution (physiological salinesolution) 108 (20 to 25° C.) was added up to the same level as the topsurface of the glass filter plate, on which a filter paper 107 having adiameter of 90 mm (produced by ADVANTEC Toyo Co., Ltd., trade name: (JISP 3801, No. 2), thickness: 0.26 mm, diameter of captured particles: 5μm) was then mounted so that its entire surface would be wetted, andfurther, an excess of liquid was removed.

The one set of measurement apparatus was mounted on the above wet filterpaper, thereby getting the liquid absorbed under the load for apredetermined duration. This absorption duration was defined as 1 hourfrom the start of the measurement. Specifically, 1 hour later, the oneset of measurement apparatus was removed by being lifted to measure itsmass Wb (g). This measurement of the mass must be carried out as quicklyas possible and so as not to give any vibration. Then, the absorptioncapacity under load (AAP) (g/g) was calculated from the Wa and Wb inaccordance with the following equation:AAP(g/g)=[Wb(g)−Wa(g)]/mass (g) of water-absorbent resin orwater-absorbent resin composition

<Saline Flow Conductivity (SFC)>:

The following test was carried out according to the saline flowconductivity (SFC) test as described in JP-A-509591/1997 (Kohyo).

An apparatus as shown in FIG. 3 was used, and a water-absorbent resin orwater-absorbent resin composition (0.900 g) as uniformly placed in areceptacle 40 was swollen in synthetic urine (1) under a load of 0.3 psi(2.07 kPa) for 60 minutes (which was 120 minutes in the case ofmeasuring the retention ratio of the saline flow conductivity (SFC)),and the gel layer height of the resultant gel 44 was recorded. Next,under the load of 0.3 psi (2.07 kPa), a 0.69 mass % aqueous sodiumchloride solution 33 was passed through the swollen gel layer from atank 31 under a constant hydrostatic pressure. This SFC test was carriedout at room temperature (20 to 25° C.). The amount of the liquid passingthrough the gel layer was recorded as a function to time with a computerand a balance at twenty seconds' intervals for 10 minutes. The rateF_(s) (t) of the flow passing through the swollen gel 44 (mainly betweenparticles thereof) was determined in a unit of g/s by dividing theincremental mass (g) by the incremental time (s). The time when theconstant hydrostatic pressure and the stable flow rate are obtained wasrepresented by t_(s), and only the data as obtained between t_(s) and 10minutes were used for the flow rate calculation. The F_(s) (t=0) value,namely, the initial rate of the flow passing through the gel layer, wascalculated from the flow rates as obtained between t_(s) and 10 minutes.The F_(s) (t=0) was calculated by extrapolating the results of aleast-squares fit of F_(s) (t) versus time to t=0.SFC=(F _(s)(t=0)×L ₀)/(ρ×A×ΔP)=(F _(s)(t=0)×L ₀)/139,506where:F_(s)(t=0): flow rate denoted by g/s;L₀: initial thickness of gel layer denoted by cm;ρ: density of NaCl solution (1.003 g/cm³);A: area of top of gel layer in cell 41 (28.27 cm²);ΔP: hydrostatic pressure applied to gel layer (4,920 dyne/cm²); and

-   -   the unit of the SFC is: (×10⁻⁷·cm³·s·g⁻¹).

As to the apparatus as shown in FIG. 3, a glass tube 32 was inserted inthe tank 31, and the lower end of the glass tube 32 was placed so thatthe 0.69 mass % aqueous sodium chloride solution 33 could be maintainedat a height of 5 cm from the bottom of the swollen gel 44 in the cell41. The 0.69 mass % aqueous sodium chloride solution 33 in the tank 31was supplied to the cell 41 through an L-tube 34 having a cock. Areceptacle 48 to collect the passed liquid was placed under the cell 41,and this collecting receptacle 48 was set on a balance 49. The innerdiameter of the cell 41 was 6 cm, and a No. 400 stainless metal gauze(mesh opening size: 38 μm) 42 was set at the bottom thereof. Holes 47sufficient for the liquid to pass through were opened in the lowerportion of a piston 46, and its bottom portion was equipped with awell-permeable glass filter 45 so that the water-absorbent resin orwater-absorbent resin composition or their swollen gels would not enterthe holes 47. The cell 41 was placed on a stand to put the cell thereon.The face, contacting with the cell, of the stand was set on a stainlessmetal gauze 43 that did not inhibit the liquid permeation.

The synthetic urine (1) as used was obtained by mixing together thefollowing: 0.25 g of calcium chloride dihydrate; 2.0 g of potassiumchloride; 0.50 g of magnesium chloride hexahydrate; 2.0 g of sodiumsulfate; 0.85 g of ammonium dihydrogenphosphate; 0.15 g of diammoniumhydrogenphosphate; and 994.25 g of pure water.

<Saline Flow Conductivity (SFC) after Paint Shaker Test (SFC after PS)>:

The following measurement was carried out on the basis of the apparatusas disclosed in JP-A-235378/1997 (Kokai).

A glass bottle of 6 cm in diameter and 11 cm in height having a lid wascharged with 30 g of water-absorbent resin or water-absorbent resincomposition and about 10 g of glass beads (diameter: 6 mm), and thenattached to a TOYOSEIKI PAINT SHAKER (for 100 V, 60 HZ), and then shakenfor 30 minutes. The glass beads and the water-absorbent resin orwater-absorbent resin composition were sieved with a metal gauze havinga mesh opening size of about 2 mm, thus obtaining a water-absorbentresin or water-absorbent resin composition after the paint shaker test.

The saline flow conductivity of the obtained water-absorbent resin orwater-absorbent resin composition was measured by the aforementionedmethod.

Incidentally, the CRC and AAP after PS can also be measured by the samemethod.

<Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation(σζ) of Particle Diameter Distribution>:

Water-absorbent resins or water-soluble polyvalent metal salt particlesor water-absorbent resin compositions were classified with JIS standardsieves having mesh opening sizes of such as 850 μm, 710 μm, 600 μm, 500μm, 425 μm, 300 μm, 212 μm, 150 μm, and 45 μm. Then, the percentages Rof the residues on these sieves were plotted on a logarithmicprobability paper. Therefrom, a particle diameter corresponding to R=50mass % was read as the mass-average particle diameter (D50). Inaddition, the logarithmic standard deviation (σζ) of the particlediameter distribution is shown by the following equation. The smaller σζvalue shows the narrower particle diameter distribution.σζ=0.5×ln(X2/X1)

(wherein: X1 is a particle diameter when R=84.1 mass %, and X2 is aparticle diameter when R=15.9 mass %)

As to the classification method for measuring the mass-average particlediameter (D50) and the logarithmic standard deviation (σζ) of theparticle diameter distribution, 10.0 g of water-absorbent resin orwater-soluble polyvalent metal salt particles or water-absorbent resincomposition was placed onto JIS standard sieves (having mesh openingsizes of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm,and 45 μm) (THE IIDA TESTING SIEVE: diameter=8 cm) under conditions of aroom temperature (20 to 25° C.) and a humidity of 50 RH %, and thenclassified with a shaking classifier (IIDA SIEVE SHAKER, TYPE: ES-65type, SER. No. 0501) for 5 minutes.

<Dust Generation Degree>:

The water-absorbent resin or water-absorbent resin composition wasplaced into a PE bag (No. 13) as beforehand coated with an antistaticagent. This bag was shaken 30 times and then opened to carry out themeasurement with DIGITAL DUST INDICATOR P-5L (produced by SHIBATA) for 1minute. This measurement was carried out 10 times, and its average valuewas determined.

<Capillary Absorption Capacity (CSF)>:

The CSF is an index showing the capillary suction force of thewater-absorbent resin or water-absorbent resin composition.

The capillary absorption capacity is determined by measuring the abilityof the absorbent structure to absorb a liquid against a negativepressure gradient of the water column of 20 cm under a load of 0.06 psiwithin a predetermined time.

While referring to FIG. 4, an apparatus and method for measuring thecapillary absorption capacity are described.

A conduit 3 was connected to a lower portion of a glass filter 2 of 60mm in diameter having a liquid-absorbing surface of a porous glass plate1 (glass filter particle No. #3: Buchner type filter TOP 17G-3 (code no.1175-03) produced by Sogo Rikagaku Glass Seisakusho Co., Ltd.), and thisconduit 3 was connected to an opening as provided to a lower portion ofa liquid storage container 4 of 10 cm in diameter. The porous glassplate of the aforementioned glass filter has an average pore diameter of20 to 30 μm, and can retain water in the porous glass plate by itscapillary force against the negative pressure of the water column evenin a state where a difference of 60 cm between heights of liquidsurfaces is made, so that a state of no introduction of air can be kept.A supporting ring 5 was fitted to the glass filter 2 in order to let upand down its height, and the system was filled with a 0.90 mass %physiological saline solution (0.90 mass % aqueous NaCl solution) (0.90%NaCl solution) 6, and the liquid storage container was put on a balance7. After it had been confirmed that there was no air in the conduit andunder the porous glass plate of the glass filter, the difference inheight between a liquid surface level of the top of the physiologicalsaline solution (0.90 mass % aqueous NaCl solution) 6 in the liquidstorage container 4 and a level of the upside of the porous glass plate1 was adjusted to 20 cm, and then the glass filter was fixed to a stand8.

An amount of 0.44 g of specimen to be measured 9 (water-absorbent resincomposition) was quickly dispersed uniformly onto the glass filter(porous glass plate 1) in the funnel, and further thereon a load 10(0.06 psi) having a diameter of 59 mm was put, and then, 30 minuteslater, there was measured a value (W20) of the 0.90 mass % physiologicalsaline solution (0.90 mass % aqueous NaCl solution) as absorbed by thespecimen to be measured 9.

The capillary absorption capacity is determined from the followingequation.Capillary absorption capacity (CSF) (g/g)=absorption amount (W20)(g)/0.44 (g)

<Retention Ratio of Saline Flow Conductivity (SFC) (Retention Ratio ofSFC)>:

In the aforementioned method for measurement of the saline flowconductivity (SFC), the swelling time under the load of 0.3 psi (2.07kPa) was changed from 60 minutes to 120 minutes, and thereafter themeasurement was carried out in the same way. The saline flowconductivity (SFC) as measured after the swelling time of 60 minutes isherein referred to as SFC (1 hr), and the saline flow conductivity (SFC)as measured after the swelling time of 120 minutes is herein referred toas SFC (2 hr). The retention ratio of the SFC is represented by thefollowing equation:Retention ratio (%) of SFC=[SFC(2 hr)/SFC(1 hr)]×100

<Retention Ratio of Saline Flow Conductivity (SFC) After Paint ShakerTest (Retention Ratio After PS)>:

The following measurement was carried out on the basis of the apparatusas disclosed in JP-A-235378/1997 (Kokai).

A glass bottle of 6 cm in diameter and about 11 cm in height having alid was charged with 30 g of water-absorbent resin composition and about10 g of glass beads (diameter: 6 mm), and then attached to a TOYOSEIKIPAINT SHAKER (for 100 V, 60 HZ), and then shaken for 30 minutes. Theglass beads and the water-absorbent resin composition were sieved with ametal gauze having a mesh opening size of about 2 mm, thus obtaining awater-absorbent resin composition after the paint shaker test. Theliquid permeation rate under load of the obtained water-absorbent resincomposition was measured by the aforementioned method. When the liquidpermeation rate under load of the water-absorbent resin compositionafter the paint shaker test is represented by SFC (after PS) and whenthe liquid permeation rate under load of the water-absorbent resincomposition before the paint shaker test is represented by SFC (beforePS), then the retention ratio after PS is represented by the followingequation:Retention ratio (%) after PS=[SFC(after PS)/SFC(before PS)]×100

<Contact Angle>:

A double-coated pressure-sensitive adhesive tape was stuck onto an SUSsheet, and then the water-absorbent resin or water-absorbent resincomposition was closely and uniformly spread onto this double-coatedtape, and then the water-absorbent resin or water-absorbent resincomposition which had not adhered to the double-coated tape was scrapedoff to prepare a specimen sheet of which the surface was covered withthe water-absorbent resin or water-absorbent resin composition. When aphysiological saline solution (0.90 mass %) was made to contact with theabove specimen sheet, the contact angle was measured by the sessile dropmethod with a contact angle meter (FACE CA-X model, produced by KyowaKaimen Kagaku K.K.) under conditions of 20° C. and 60% RH. The contactangle at 1 second later than dropping a liquid drop of the physiologicalsaline solution onto the specimen sheet was measured 5 times per onespecimen. Its average value was determined and taken as the contactangle of the water-absorbent resin or water-absorbent resin composition.

<Extractable (Water-Extractable) Component Content>:

Into a plastic receptacle of 250 ml in capacity having a lid, 184.3 g of0.90 mass % physiological saline solution was weighed out. Then, 1.00 gof water-absorbent resin particles or water-absorbent resin compositionwas added to this aqueous solution, and they were stirred for 16 hours,thereby the extractable component content in the resin was extracted.This extract liquid was filtrated with a filter paper (produced byADVANTEC Toyo Co., Ltd., trade name: (JIS P 3801, No. 2), thickness:0.26 mm, diameter of captured particles: 5 μm), and then 50.0 g of theresultant filtrate was weighed out and used as a measuring solution.

To begin with, only the 0.90 mass % physiological saline solution wasfirstly titrated with an aqueous 0.1N NaOH solution until the pH reached10, and then the resultant solution was titrated with an aqueous 0.1NHCl solution until the pH reached 2.7, thus obtaining blank titrationamounts ([bNaOH] ml and [bHCl] ml).

The same titration procedure was carried out for the measuring solution,thus obtaining titration amounts ([NaOH] ml and [HCl] ml).

For example, if the water-absorbent resin or water-absorbent resinparticles or water-absorbent resin composition comprises acrylic acidand its sodium salt in known amounts, the extractable component contentof the water-absorbent resin can be calculated from the averagemolecular weight of the monomers and the titration amounts, as obtainedfrom the above procedures, in accordance with the following equation. Inthe case of unknown amounts, the average molecular weight of themonomers is calculated from the neutralization degree as determined bythe titration.Extractable component content (mass %)=0.1×(average molecularweight)×184.3×10×([HCl]−[bHCl])/1,000/1.0/50.0Neutralization degree (mol %)=(1−([NaOH]−[bNaOH])/([HCl]−[bHCl]))×100

<Quantification of Metal Compound>:

Appropriate publicly known methods are used for the quantification ofthe metal compound. For example, the quantification is carried out bysuch as atomic absorption photometry, plasma emission spectrometry, andabsorption photometry which uses color reaction reagents. When it comesto the water-soluble metal compounds, the plasma emission spectrometryis favorably used. Hereinafter an example of the measurement methods iscited.

An amount of 1.0 g of water-absorbent resin composition was weighed outinto a polypropylene-made beaker of 260 ml in capacity, and then thereto190.0 g of 0.90 mass % physiological saline solution and 10.0 g of 2Nhydrochloric acid were added, and then they were stirred at roomtemperature for 30 minutes. After the stirring, the resultantsupernatant was filtered with a chromatodisk (GL Chromatodisk 25A of GLScience). The filtrate was analyzed by plasma emission spectrometry(with ULTIMA, produced by Horiba Seisakusho) to determine the metal saltconcentration. Incidentally, the calibration curve was prepared from a0.90 mass % physiological saline solution containing a known amount ofpolyvalent metal. Based on the determined polyvalent metalconcentration, the polyvalent metal concentration in the water-absorbentresin composition is shown by the following equation:Polyvalent metal concentration (mass %) in water-absorbent resincomposition=(polyvalent metal concentration (mass %) in solution)×200

<Metal Compound Segregation Index>:

This is a numerical value indicating the tendency for the segregation ofthe metal compound to occur and is measured by the following method.

The metal compound as contained in the water-absorbent resin compositionis quantified by the aforementioned method and calculated from thefollowing equation:Metal compound segregation index=W71 (ppm)/W72 (ppm)

The W71 represents a concentration (ppm) of a metal component which is,for example, contained in a water-absorbent resin composition havingpassed through a JIS standard sieve of 300 μm in mesh opening size (aportion having particle diameters of not larger than 300 μm) by carryingout the classification operation in the same way as of theaforementioned measurement of the mass-average particle diameter. Inaddition, the W72 represents a concentration (ppm) of the metalcomponent which is contained in a water-absorbent resin compositionbefore the classification operation. The mesh opening size of the sievein the present measurement method is changed appropriately for themass-average particle diameter of the water-absorbent resin composition.Concretely, it is specified as follows: when the mass-average particlediameter is not larger than 150 μm, a JIS standard sieve of 45 μm inmesh opening size should be used; when the mass-average particlediameter is in the range of 150 to 300 μm, a JIS standard sieve of 150μm in mesh opening size should be used; when the mass-average particlediameter is in the range of 300 to 500 μm, a JIS standard sieve of 300μm in mesh opening size should be used; and, when the mass-averageparticle diameter is not smaller than 500 μm, a JIS standard sieve of500 μm in mesh opening size should be used.

<Blocking Ratio (BR)>:

This refers to a blocking ratio when having been put at 25° C. and arelative humidity of 70% for 1 hour.

An amount of 2 g of water-absorbent resin particles (or water-absorbentresin composition) is uniformly spread onto a bottom of apolypropylene-made cup of 50 mm in inner diameter of the bottom and 10mm in height and then quickly placed into a thermohumidistatic incubator(PLATIOOUS LUCIFER PL-2G, produced by Tabai Espec Co., Ltd.) (which hadbeforehand been adjusted to 25° C. and the relative humidity of 70%) andthen left alone for 60 minutes. Thereafter, the water-absorbent resinparticles having absorbed the moisture were transferred onto a JISstandard sieve of 7.5 cm in diameter and 2,000 μm in mesh opening sizeand then sieved with a shaking classifier (IIDA SIEVE SHAKER, TYPE:ES-65 type, SER. No. 0501) for 5 minutes. Then, mass W4 (g) ofwater-absorbent resin particles remaining on the sieve and mass W5 (g)of water-absorbent resin particles having passed through the sieve weremeasured.

The moisture absorption blocking ratio (%) was calculated from thefollowing equation:Moisture absorption blocking ratio (%)=mass W4 (g)/(mass W4 (g)+mass W5(g))×100

The lower the moisture absorption blocking ratio is, the more excellentthe moisture absorption flowability is.

As to the below-mentioned experiments for demonstrating the effects ofadding the metal compound, it is favorable that the same precursor isused to compare the above effects. For example, if the particle diameterdistribution of the precursor varies, there is a possibility that theparameters depending on the particle diameter distribution, such as SFC,cannot precisely be evaluated. For example, when the performance, asindicated by the SFC, of water-absorbent resin particles is compared, itis favorable to compare the SFC using water-absorbent resin particleshaving almost the same CRC and particle diameter distributions.

REFERENTIAL EXAMPLE 1

In a reactor as prepared by lidding a jacketed stainless twin-armkneader of 10 liters in capacity having two sigma-type blades, there wasprepared a reaction liquid by dissolving 11.7 g (0.10 mol %) ofpolyethylene glycol diacrylate into 5,438 g of aqueous solution ofsodium acrylate having a neutralization degree of 71.3 mol % (monomerconcentration: 39 mass %). Next, this reaction liquid was deaeratedunder an atmosphere of nitrogen gas for 30 minutes. Subsequently, 29.34g of 10 mass % aqueous sodium persulfate solution and 24.45 g of 0.1mass % aqueous L-ascorbic acid solution were added thereto under stirredconditions. As a result, polymerization started after about 1 minute.Then, the polymerization was carried out in the range of 20 to 95° C.while the forming gel was pulverized. Then, the resultant crosslinkedhydrogel polymer was taken out after 30 minutes from the start of thepolymerization. The crosslinked hydrogel polymer as obtained above wasin the form of finely divided pieces having diameters of not larger thanabout 5 mm. This finely divided crosslinked hydrogel polymer was spreadonto a metal gauze of 50 meshes (mesh opening size: 300 μm) and thenhot-air-dried at 180° C. for 50 minutes, thus obtaining awater-absorbent resin (1) which was of the irregular shape and easy topulverize, such as in the form of particles, a powder, or a particulatedried material agglomerate.

The resultant water-absorbent resin (1) was pulverized with a roll milland then further classified with a JIS standard sieve having a meshopening size of 850 μm. Next, particles having passed through the 850 μmin the aforementioned operation were classified with a JIS standardsieve having a mesh opening size of 150 μm, whereby a water-absorbentresin (1aF) passing through the JIS standard sieve having the meshopening size of 150 μm was removed, thus obtaining a particulatewater-absorbent resin (1a).

The resultant water-absorbent resin (1aF) was agglomerated according tothe method of Granulation Example 1 as disclosed in U.S. Pat. No.6,228,930. The resultant agglomerated material was pulverized andclassified by the same procedure as the aforementioned, thus obtainingan agglomerated water-absorbent resin (1aA).

An amount of 90 mass parts of the water-absorbent resin (1a) and 10 massparts of the water-absorbent resin (1aA), as obtained in the above way,were uniformly mixed together to obtain a water-absorbent resin (A).

In addition, similarly, the resultant water-absorbent resin (1) waspulverized with a roll mill and then further classified with a JISstandard sieve having a mesh opening size of 710 μm. Next, particleshaving passed through the 710 μm in the aforementioned operation wereclassified with a JIS standard sieve having a mesh opening size of 150μm, whereby water-absorbent resin particles (1bF) passing through theJIS standard sieve having the mesh opening size of 150 μm were removed,thus obtaining a particulate water-absorbent resin (1b).

The resultant water-absorbent resin (1bF) was agglomerated according tothe method of Granulation Example 1 as disclosed in U.S. Pat. No.6,228,930. The resultant agglomerated material was pulverized andclassified by the same procedure as the aforementioned, thus obtainingan agglomerated water-absorbent resin (1bA).

An amount of 85 mass parts of the water-absorbent resin (1b) and 15 massparts of the water-absorbent resin (1bA), as obtained in the above way,were uniformly mixed together to obtain a water-absorbent resin (B).

In addition, similarly, the resultant water-absorbent resin (1) waspulverized with a roll mill and then further classified with a JISstandard sieve having a mesh opening size of 600 μm. Next, particleshaving passed through the 600 μm in the aforementioned operation wereclassified with a JIS standard sieve having a mesh opening size of 150μm, whereby water-absorbent resin particles (1cF) passing through theJIS standard sieve having the mesh opening size of 150 μm were removed,thus obtaining a particulate water-absorbent resin (1c).

The resultant water-absorbent resin (1cF) was agglomerated according tothe method of Granulation Example 1 as disclosed in U.S. Pat. No.6,228,930. The resultant agglomerated material was pulverized andclassified by the same procedure as the aforementioned, thus obtainingan agglomerated water-absorbent resin (1cA).

An amount of 80 mass parts of the water-absorbent resin (1c) and 20 massparts of the water-absorbent resin (1cA), as obtained in the above way,were uniformly mixed together to obtain a water-absorbent resin (C).

REFERENTIAL EXAMPLE 2

An amount of 100 g of the water-absorbent resin (C) as obtained from theaforementioned Referential Example 1 was mixed with a surface-treatingagent comprising a mixed liquid of 0.5 g of 1,4-butanediol, 1.0 g ofpropylene glycol, and 3.0 g of pure water, and then the resultantmixture was heat-treated at 210° C. for 30 minutes. Furthermore, theresultant particles were disintegrated to such a degree that they couldpass through a JIS standard sieve having a mesh opening size of 600 μm.As a result, a surface-crosslink-treated water-absorbent resin (C1) wasobtained.

The results of having measured the properties of the water-absorbentresin (C1) are shown in Table 1.

REFERENTIAL EXAMPLE 3

An amount of 100 g of the water-absorbent resin (A) as obtained from theaforementioned Referential Example 1 was mixed with a surface-treatingagent comprising a mixed liquid of 0.1 g of 2-ethyloxetane, 3.0 g ofpure water, and 0.3 g of 24 mass % aqueous sodium hydroxide solution,and then the resultant mixture was heat-treated at 200° C. for 30minutes. Furthermore, the resultant particles were disintegrated to sucha degree that they could pass through a JIS standard sieve having a meshopening size of 850 μm. As a result, a surface-crosslink-treatedwater-absorbent resin (A1) was obtained.

The results of having measured the properties of the water-absorbentresin (A1) are shown in Table 1.

REFERENTIAL EXAMPLE 4

An amount of 100 g of the water-absorbent resin (B) as obtained from theaforementioned Referential Example 1 was mixed with a surface-treatingagent comprising a mixed liquid of 0.5 g of 2-oxazolidinone, 1.0 g ofpropylene glycol, and 4.0 g of pure water, and then the resultantmixture was heat-treated at 190° C. for 30 minutes. Furthermore, theresultant particles were disintegrated to such a degree that they couldpass through a JIS standard sieve having a mesh opening size of 710 μm.As a result, a surface-crosslink-treated water-absorbent resin (B1) wasobtained.

The results of having measured the properties of the water-absorbentresin (B1) are shown in Table 1.

REFERENTIAL EXAMPLE 5

An amount of 100 g of the water-absorbent resin (C) as obtained from theaforementioned Referential Example 1 was mixed with a surface-treatingagent comprising a mixed liquid of 0.5 g of ethylene carbonate, 1.0 g ofpropylene glycol, and 4.0 g of pure water, and then the resultantmixture was heat-treated at 195° C. for 30 minutes. Furthermore, theresultant particles were disintegrated to such a degree that they couldpass through a JIS standard sieve having a mesh opening size of 600 μm.As a result, a surface-crosslink-treated water-absorbent resin (C2) wasobtained.

The results of having measured the properties of the water-absorbentresin (C2) are shown in Table 1.

REFERENTIAL EXAMPLE 6

An amount of 100 g of the water-absorbent resin (A) as obtained from theaforementioned Referential Example 1 was mixed with a surface-treatingagent comprising a mixed liquid of 0.5 g of ethylene carbonate, 6.0 g ofpure water, and 0.5 g of aluminum sulfate tetradeca- to octadecahydrates(obtained from Kanto Chemical Co., Inc.), and then the resultant mixturewas heat-treated at 195° C. for 30 minutes. Furthermore, the resultantparticles were disintegrated to such a degree that they could passthrough a JIS standard sieve having a mesh opening size of 850 μm. As aresult, a surface-crosslink-treated water-absorbent resin (A2) wasobtained.

The results of having measured the properties of the water-absorbentresin (A2) are shown in Table 1.

REFERENTIAL EXAMPLE 7

Aluminum sulfate tetradeca- to octadecahydrates (obtained from KantoChemical Co., Inc.) were classified with JIS standard sieves having meshopening sizes of 600 μm, 300 μm, and 150 μm, thus obtaining aluminumsulfate tetradeca- to octadecahydrates (1) having particle diameters ofsubstantially not larger than 150 μm, aluminum sulfate tetradeca- tooctadecahydrates (2) having particle diameters of substantially 300 to150 μm, and aluminum sulfate tetradeca- to octadecahydrates (3) havingparticle diameters of substantially 600 to 300 μm. The mass-averageparticle diameter of the aluminum sulfate tetradeca- to octadecahydrates(1) was 95 μm, the mass-average particle diameter of the aluminumsulfate tetradeca- to octadecahydrates (2) was 203 μm, and themass-average particle diameter of the aluminum sulfate tetradeca- tooctadecahydrates (3) was 401 μm.

EXAMPLE 1

An amount of 100 mass parts of the water-absorbent resin (C1), asobtained from Referential Example 2, was uniformly mixed with 0.5 masspart of aluminum sulfate hydrates (trideca- to tetradecahydrates,obtained from Sumitomo Chemical Co., Ltd., mass-average particlediameter: 165 μm, bulk density: 0.86 g/cm³, solubility into pure waterof 0° C.: 46.4 mass %), thus obtaining a water-absorbent resincomposition (1).

The results of having measured the properties of the resultantwater-absorbent resin composition (1) are shown in Table 1. In addition,the result of having further measured the CSF is shown in Table 4.

EXAMPLE 2

An amount of 100 mass parts of the water-absorbent resin (C1), asobtained from Referential Example 2, was uniformly mixed with 1.0 masspart of aluminum sulfate tetradeca- to octadecahydrates (obtained fromKanto Chemical Co., Inc., mass-average particle diameter: 182 μm, bulkdensity: 0.60 g/cm³), thus obtaining a water-absorbent resin composition(2).

The results of having measured the properties of the resultantwater-absorbent resin composition (2) are shown in Table 1. In addition,the result of having further measured the retention ratio of the salineflow conductivity (SFC) is shown in Table 2, and the result of havingfurther measured the CSF is shown in Table 4.

EXAMPLE 3

An amount of 100 mass parts of the water-absorbent resin (A1), asobtained from Referential Example 3, was uniformly mixed with 1.0 masspart of aluminum sulfate hydrates (trideca- to tetradecahydrates,obtained from Sumitomo Chemical Co., Ltd., mass-average particlediameter: 165 μm, bulk density: 0.86 g/cm³), thus obtaining awater-absorbent resin composition (3).

The results of having measured the properties of the resultantwater-absorbent resin composition (3) are shown in Table 1.

EXAMPLE 4

An amount of 100 mass parts of the water-absorbent resin (B1), asobtained from Referential Example 4, was uniformly mixed with 0.1 masspart of aluminum sulfate hydrates (trideca- to tetradecahydrates,obtained from Sumitomo Chemical Co., Ltd., mass-average particlediameter: 165 μm, bulk density: 0.86 g/cm³), thus obtaining awater-absorbent resin composition (4).

The results of having measured the properties of the resultantwater-absorbent resin composition (4) are shown in Table 1.

EXAMPLE 5

An amount of 100 mass parts of the water-absorbent resin (C2), asobtained from Referential Example 5, was uniformly mixed with 0.5 masspart of aluminum sulfate hydrates (trideca- to tetradecahydrates,obtained from Sumitomo Chemical Co., Ltd., mass-average particlediameter: 165 μm, bulk density: 0.86 g/cm³), thus obtaining awater-absorbent resin composition (5).

The results of having measured the properties of the resultantwater-absorbent resin composition (5) are shown in Table 1.

EXAMPLE 6

An amount of 100 mass parts of the water-absorbent resin (C1), asobtained from Referential Example 2, was uniformly mixed with 0.5 masspart of the aluminum sulfate tetradeca- to octadecahydrates (1) asobtained from Referential Example 7, thus obtaining a water-absorbentresin composition (6).

The results of having measured the properties of the resultantwater-absorbent resin composition (6) are shown in Table 1.

EXAMPLE 7

An amount of 100 mass parts of the water-absorbent resin (C1), asobtained from Referential Example 2, was uniformly mixed with 0.5 masspart of the aluminum sulfate tetradeca- to octadecahydrates (2) asobtained from Referential Example 7, thus obtaining a water-absorbentresin composition (7).

The results of having measured the properties of the resultantwater-absorbent resin composition (7) are shown in Table 1.

EXAMPLE 8

An amount of 100 mass parts of the water-absorbent resin (C1), asobtained from Referential Example 2, was uniformly mixed with 0.5 masspart of the aluminum sulfate tetradeca- to octadecahydrates (3) asobtained from Referential Example 7, thus obtaining a water-absorbentresin composition (8).

The results of having measured the properties of the resultantwater-absorbent resin composition (8) are shown in Table 1.

EXAMPLE 9

The wettability to a physiological saline solution was evaluated by thecontact angle as to water-absorbent resin compositions.

The contact angle with the physiological saline solution was measured bythe aforementioned method as to the water-absorbent resin composition(1) as obtained from Example 1 (aluminum sulfate-added material) (beforethe PS) and as to a water-absorbent resin composition as obtained in thesame way as of Example 1 except the aluminum sulfate was replaced withAerosil R-972 (produced by Nippon Aerosil Co., Ltd.).

The results of having measured the contact angle are shown in Table 3.

COMPARATIVE EXAMPLE 1

The water-absorbent resin (C1), as obtained from Referential Example 2,was taken as a comparative water-absorbent resin (1).

The results of having measured the properties of the resultantcomparative water-absorbent resin (1) are shown in Table 1.

COMPARATIVE EXAMPLE 2

An amount of 100 mass parts of the water-absorbent resin (A), asobtained from Referential Example 1, was uniformly mixed with 0.5 masspart of aluminum sulfate tetradeca- to octadecahydrates (obtained fromKanto Chemical Co., Inc., mass-average particle diameter: 182 μm, bulkdensity: 0.60 g/cm³), thus obtaining a comparative water-absorbent resincomposition (2).

The results of having measured the properties of the resultantcomparative water-absorbent resin composition (2) are shown in Table 1.In addition, the result of having further measured the CSF is shown inTable 4.

COMPARATIVE EXAMPLE 3

An amount of 100 mass parts of the water-absorbent resin (A), asobtained from Referential Example 1, was uniformly mixed with 5 massparts of 10 mass % aqueous solution of aluminum sulfate tetradeca- tooctadecahydrates (obtained from Kanto Chemical Co., Inc., mass-averageparticle diameter: 182 μm, bulk density: 0.60 g/cm³, solubility intopure water of 23° C.: 37.5 mass %), thus obtaining a comparativewater-absorbent resin composition (3).

The results of having measured the properties of the resultantcomparative water-absorbent resin composition (3) are shown in Table 1.In addition, the result of having further measured the CSF is shown inTable 4.

COMPARATIVE EXAMPLE 4

An amount of 100 mass parts of the water-absorbent resin (C1), asobtained from Referential Example 2, was uniformly mixed with 5 massparts of 10 mass % aqueous solution of aluminum sulfate tetradeca- tooctadecahydrates (obtained from Kanto Chemical Co., Inc., mass-averageparticle diameter: 182 μm, bulk density: 0.60 g/cm³), thus obtaining acomparative water-absorbent resin composition (4).

The results of having measured the properties of the resultantcomparative water-absorbent resin composition (4) are shown in Table 1.In addition, the result of having further measured the retention ratioof the saline flow conductivity (SFC) is shown in Table 2.

COMPARATIVE EXAMPLE 5

The water-absorbent resin (A2), as obtained from Referential Example 6,was taken as a comparative water-absorbent resin (5).

The results of having measured the properties of the resultantcomparative water-absorbent resin (5) are shown in Table 1. In addition,the result of having further measured the CSF is shown in Table 4.

COMPARATIVE EXAMPLE 6

An amount of 100 mass parts of the water-absorbent resin (C2), asobtained from Referential Example 5, was uniformly mixed with 0.5 masspart of aluminum sulfate hydrates (trideca- to tetradecahydrates,obtained from Sumitomo Chemical Co., Ltd., mass-average particlediameter: 165 μm, bulk density: 0.86 g/cm³), and then adding 5 massparts of pure water to the resultant mixture, thus obtaining acomparative water-absorbent resin composition (6).

The results of having measured the properties of the resultantcomparative water-absorbent resin composition (6) are shown in Table 1.

EXAMPLE 10

Examinations with electron photomicrographs were made about the statesof the aluminum sulfates in the water-absorbent resin composition (2) asobtained from Example 2, the comparative water-absorbent resincomposition (4) as obtained from Comparative Example 4, and thecomparative water-absorbent resin composition (6) as obtained fromComparative Example 6.

In the water-absorbent resin composition (2), the aluminum sulfate waspresent in the form of particles. However, in the comparativewater-absorbent resin compositions (4) and (6), the aluminum sulfate waspartially or entirely dissolved and was therefore not present in theform of particles. Incidentally, the presence of the aluminum sulfatewas confirmed by EPMA analysis into aluminum.

TABLE 1 Mass- Water- average absorbent particle SFC (before Retentionresin as diameter CRC AAP PS) SFC (after PS) ratio after precursor μmg/g g/g (×10⁻⁷ · cm³ · s · g⁻¹) (×10⁻⁷ · cm³ · s · g⁻¹) PS % ReferentialC Water- 302 26 25 72 — — Example 2 (600-150 μm) absorbent resin (C1)Referential A Water- 435 30 26 42 — — Example 3 (850-150 μm) absorbentresin (A1) Referential B Water- 362 24 23 121 — — Example 4 (710-150 μm)absorbent resin (B1) Referential C Water- 301 28 25 54 — — Example 5(600-150 μm) absorbent resin (C2) Referential A Water- 437 29 23 50 — —Example 6 (850-150 μm) absorbent resin (A2) Example 1 C1 Water- 300 2624 150 121 81 absorbent resin composition (1) Example 2 C1 Water- 304 2624 168 132 79 absorbent resin composition (2) Example 3 A1 Water- 432 3025 63 45 71 absorbent resin composition (3) Example 4 B1 Water- 359 2422 186 135 73 absorbent resin composition (4) Example 5 C2 Water- 297 2824 102 73 72 absorbent resin composition (5) Example 6 C1 Water- 298 2624 156 128 82 absorbent resin composition (6) Example 7 C1 Water- 301 2624 140 134 96 absorbent resin composition (7) Example 8 C1 Water- 305 2624 128 131 102  absorbent resin composition (8) Comparative C1Comparative 302 26 25 72 42 58 Example 1 water- absorbent resin (1)Comparative A Comparative 431 33 11 3 — — Example 2 water- absorbentresin composition (2) Comparative A Comparative 430 33 10 2 — — Example3 water- absorbent resin composition (3) Comparative C1 Comparative 30125 21 112 46 41 Example 4 water- absorbent resin composition (4)Comparative A2 Comparative 437 29 22 50 23 46 Example 5 water- absorbentresin (5) Comparative C2 Comparative 314 27 23 98 43 44 Example 6 water-absorbent resin composition (6)

TABLE 2 Water- SFC (1 hr) SFC (2 hr) Retention absorbent (×10⁻⁷ · cm³ ·(×10⁻⁷ · cm³ · ratio of resin s · g⁻¹) s · g⁻¹) SFC % Example C1 Water-168 112 67 2 absorbent resin composition (2) Compar- C1 Comparative 112 43 38 ative water- Example absorbent 4 resin composition (4)

TABLE 3 Contact angle Inorganic compound added (degrees) Aluminumsulfate tetradeca- 16.9 to octadecahydrates Aerosil R-972 87.7 Note)Aerosil R-972: hydrophobic silica, produced by Nippon Aerosil Co., Ltd.in dry manner

TABLE 4 CSF g/g Example 1 Water- 23 absorbent resin composition (1)Example 2 Water- 22 absorbent resin composition (2) ComparativeComparative  7 Example 2 water- absorbent resin composition (2)Comparative Comparative  5 Example 3 water- absorbent resin composition(3) Comparative Comparative 18 Example 5 water- absorbent resin (5)

REFERENTIAL EXAMPLE 8

In a reactor as prepared by lidding a jacketed stainless twin-armkneader of 10 liters in capacity having two sigma-type blades, there wasprepared a reaction liquid by dissolving 11.7 g (0.10 mol %) ofpolyethylene glycol diacrylate into 5,438 g of aqueous solution ofsodium acrylate having a neutralization degree of 71.3 mol % (monomerconcentration: 39 mass %). Next, this reaction liquid was deaeratedunder an atmosphere of nitrogen gas for 30 minutes. Subsequently, 29.34g of 10 mass % aqueous sodium persulfate solution and 24.45 g of 0.1mass % aqueous L-ascorbic acid solution were added thereto under stirredconditions. As a result, polymerization started after about 1 minute.Then, the polymerization was carried out in the range of 20 to 95° C.while the forming gel was pulverized. Then, the resultant crosslinkedhydrogel polymer was taken out after 30 minutes from the start of thepolymerization. The crosslinked hydrogel polymer as obtained above wasin the form of finely divided pieces having diameters of not larger thanabout 5 mm. This finely divided crosslinked hydrogel polymer was spreadonto a metal gauze of 50 meshes (mesh opening size: 300 μm) and thenhot-air-dried at 180° C. for 50 minutes, thus obtaining awater-absorbent resin (1) which was of the irregular shape and easy topulverize, such as in the form of a particulate dried materialagglomerate.

The resultant water-absorbent resin (1) was pulverized with a roll milland then further classified with a JIS standard sieve having a meshopening size of 850 μm. Next, particles having passed through the 850 μmin the aforementioned operation were classified with a JIS standardsieve having a mesh opening size of 150 μm, whereby a water-absorbentresin (1aF) passing through the JIS standard sieve having the meshopening size of 150 μm was removed, thus obtaining a particulatewater-absorbent resin (1a).

The removed water-absorbent resin (1aF) was agglomerated according tothe method of Granulation Example 1 as disclosed in U.S. Pat. No.6,228,930. The resultant agglomerated material was pulverized andclassified by the same procedure as the aforementioned, thus obtainingan agglomerated water-absorbent resin (1aA).

An amount of 80 mass parts of the water-absorbent resin (1a) and 20 massparts of the water-absorbent resin (1aA), as obtained in the above way,were uniformly mixed together to obtain a water-absorbent resin (1C).

Next, 500 g of the water-absorbent resin (1C) and a surface-treatingagent comprising a mixed liquid of 2.5 g of 1,4-butanediol, 5.0 g ofpropylene glycol, and 15.0 g of pure water were mixed together, and thenthe resultant mixture was heat-treated at 210° C. for 30 minutes. Theheat-treated water-absorbent resin was disintegrated to such a degreethat it could pass through a JIS standard sieve having a mesh openingsize of 600 μm. As a result, a surface-crosslinked water-absorbent resin(1D) was obtained.

Various properties of the surface-crosslinked water-absorbent resin (1D)are shown in Table 5.

The dust generation degree of the surface-crosslinked water-absorbentresin (1D) is shown in Table 6.

EXAMPLE 11

An amount of 300 g of the surface-crosslinked water-absorbent resin(1D), as obtained from Referential Example 8, was preheated to 60° C.,and then spraywise mixed with 1.5 g of water under stirring by a Lödigemixer. Subsequently, 3 g of aluminum sulfate tetradeca- tooctadecahydrates was added thereto to mix them together under stirringby the Lödige mixer, and then the resultant mixture was left alone atroom temperature for 30 minutes. The resultant mixture was disintegratedto such a degree that it could pass through a JIS standard sieve havinga mesh opening size of 600 μm. As a result, a water-absorbent resincomposition (11) was obtained.

Various properties of the water-absorbent resin composition (11) areshown in Table 5.

The dust generation degree of the water-absorbent resin composition (11)is shown in Table 6.

EXAMPLES 12 TO 14

Water-absorbent resin compositions (12) to (14) were obtained in thesame way as of Example 11 except that the amount of water being used waschanged to 3 g, 4.5 g, and 6 g respectively.

Various properties of the water-absorbent resin compositions (12) to(14) are shown in Table 5.

The dust generation degrees of the water-absorbent resin compositions(12) to (14) are shown in Table 6.

EXAMPLE 15

A water-absorbent resin composition (15) was obtained in the same way asof Example 11 except that the 1.5 g of water was replaced with 6 g ofaqueous solution of water/glycerol=50/50 (wt/wt).

Various properties of the water-absorbent resin composition (15) areshown in Table 5.

EXAMPLE 16

A water-absorbent resin composition (16) was obtained in the same way asof Example 11 except that the 1.5 g of water was replaced with 6 g ofaqueous solution of water/propylene glycol=50/50 (wt/wt).

Various properties of the water-absorbent resin composition (16) areshown in Table 5.

EXAMPLE 17

A water-absorbent resin composition (17) was obtained in the same way asof Example 11 except that the 1.5 g of water was replaced with 6 g ofaqueous solution of water/polyethylene glycol (average molecular weight:600)=50/50 (wt/wt).

Various properties of the water-absorbent resin composition (17) areshown in Table 5.

COMPARATIVE EXAMPLE 7

An amount of 300 g of the surface-crosslinked water-absorbent resin(1D), as obtained from Referential Example 8, and 3 g of aluminumsulfate tetradeca- to octadecahydrates were mixed (dry-blended) togetherunder stirring by a Lödige mixer, thus obtaining a comparativewater-absorbent resin composition (7).

Various properties of the comparative water-absorbent resin composition(7) are shown in Table 5.

EXAMPLE 18

An amount of 500 g of the not yet surface-crosslinked water-absorbentresin (1C), as obtained from Referential Example 8, was preheated to 50°C., and then mixed with a surface-treating agent comprising a mixedliquid of 2.5 g of 1,4-butanediol, 5.0 g of propylene glycol, and 15.0 gof pure water by a Lödige mixer. Subsequently, 3 g of aluminum sulfatetetradeca- to octadecahydrates was added thereto to mix them togetherunder stirring by the Lödige mixer. The resultant mixture washeat-treated at 210° C. for 30 minutes and then disintegrated to such adegree that it could pass through a JIS standard sieve having a meshopening size of 600 μm. As a result, a water-absorbent resin composition(18) was obtained.

Various properties of the water-absorbent resin composition (18) areshown in Table 5.

COMPARATIVE EXAMPLE 8

An amount of 500 g of the not yet surface-crosslinked water-absorbentresin (1C), as obtained from Referential Example 8, and asurface-treating agent comprising a mixed liquid of 2.5 g of1,4-butanediol, 5.0 g of propylene glycol, 3 g of aluminum sulfatetetradeca- to octadecahydrates, and 15.0 g of pure water were mixedtogether by a Lödige mixer. The resultant mixture was heat-treated at210° C. for 30 minutes and then disintegrated to such a degree that itcould pass through a JIS standard sieve having a mesh opening size of600 μm. As a result, a comparative water-absorbent resin composition (8)was obtained.

Various properties of the comparative water-absorbent resin composition(8) are shown in Table 5.

COMPARATIVE EXAMPLE 9

An amount of 500 g of the surface-crosslinked water-absorbent resin(1D), as obtained from Referential Example 8, was spraywise mixed with50 g of 12 mass % aqueous solution of aluminum sulfate tetradeca- tooctadecahydrates under stirring by a Lödige mixer. Next, the resultantmixture was dried at 80° C. for 30 minutes. The resultant dried materialwas disintegrated to such a degree that it could pass through a JISstandard sieve having a mesh opening size of 600 μm. As a result, acomparative water-absorbent resin composition (9) was obtained.

Various properties of the comparative water-absorbent resin composition(9) are shown in Table 5.

COMPARATIVE EXAMPLE 10

An amount of 500 g of the comparative water-absorbent resin composition(7), as obtained from Comparative Example 7, was spraywise mixed with 10g of water by a Lödige mixer and then left alone at room temperature for30 minutes. The resultant mixture was disintegrated to such a degreethat it could pass through a JIS standard sieve having a mesh openingsize of 600 μm. As a result, a comparative water-absorbent resincomposition (10) was obtained.

Various properties of the comparative water-absorbent resin composition(10) are shown in Table 5.

EXAMPLE 19

A water-absorbent resin composition (19) was obtained in the same way asof Example 13 except that aluminum sulfate tetradeca- tooctadecahydrates having passed through a JIS standard sieve having amesh opening size of 105 μm was used.

Various properties of the water-absorbent resin composition (19) areshown in Table 5.

REFERENTIAL EXAMPLE 9

First of all, a water-absorbent resin (2), which was of the irregularshape and easy to pulverize, such as in the form of a particulate driedmaterial agglomerate, was obtained in the same way as of ReferentialExample 8 except that the polyethylene glycol diacrylate was used in anamount of 7.6 g.

Specifically, in a reactor as prepared by lidding a jacketed stainlesstwin-arm kneader of 10 liters in capacity having two sigma-type blades,there was prepared a reaction liquid by dissolving 7.6 g (0.065 mol %)of polyethylene glycol diacrylate into 5,438 g of aqueous solution ofsodium acrylate having a neutralization degree of 71.3 mol % (monomerconcentration: 39 mass %). Next, this reaction liquid was deaeratedunder an atmosphere of nitrogen gas for 30 minutes. Subsequently, 29.34g of 10 mass % aqueous sodium persulfate solution and 24.45 g of 0.1mass % aqueous L-ascorbic acid solution were added thereto under stirredconditions. As a result, polymerization started after about 1 minute.Then, the polymerization was carried out in the range of 20 to 95° C.while the forming gel was pulverized. Then, the resultant crosslinkedhydrogel polymer was taken out after 30 minutes from the start of thepolymerization. The crosslinked hydrogel polymer as obtained above wasin the form of finely divided pieces having diameters of not larger thanabout 5 mm. This finely divided crosslinked hydrogel polymer was spreadonto a metal gauze of 50 meshes (mesh opening size: 300 μm) and thenhot-air-dried at 180° C. for 50 minutes, thus obtaining thewater-absorbent resin (2) which was of the irregular shape and easy topulverize, such as in the form of a particulate dried materialagglomerate.

The resultant water-absorbent resin (2) was pulverized with a roll milland then further classified with a JIS standard sieve having a meshopening size of 850 μm. Next, particles having passed through the 850 μmin the aforementioned operation were classified with a JIS standardsieve having a mesh opening size of 150 μm, whereby a water-absorbentresin (2aF) passing through the JIS standard sieve having the meshopening size of 150 μm was removed, thus obtaining a particulatewater-absorbent resin (2a).

The removed water-absorbent resin (2aF) was agglomerated according tothe method of Granulation Example 1 as disclosed in U.S. Pat. No.6,228,930. The resultant agglomerated material was pulverized andclassified by the same procedure as the aforementioned, thus obtainingan agglomerated water-absorbent resin (2aA).

An amount of 90 mass parts of the water-absorbent resin (2a) and 10 massparts of the water-absorbent resin (2aA), as obtained in the above way,were uniformly mixed together to obtain a water-absorbent resin (2C).

Next, 500 g of the water-absorbent resin (2C) and a surface-treatingagent comprising a mixed liquid of 2.5 g of 1,4-butanediol, 5.0 g ofpropylene glycol, and 15.0 g of pure water were mixed together, and thenthe resultant mixture was heat-treated at 210° C. for 30 minutes. Theheat-treated water-absorbent resin was disintegrated to such a degreethat it could pass through a JIS standard sieve having a mesh openingsize of 850 μm. As a result, a surface-crosslinked water-absorbent resin(2D) was obtained.

Various properties of the surface-crosslinked water-absorbent resin (2D)are shown in Table 5.

EXAMPLE 20

A water-absorbent resin composition (20) was obtained in the same way asof Example 13 except that the surface-crosslinked water-absorbent resin(1D) was replaced with the surface-crosslinked water-absorbent resin(2D) as obtained from Referential Example 9.

Various properties of the water-absorbent resin composition (20) areshown in Table 5.

EXAMPLE 21

The distribution of the aluminum sulfate, which was contained everyparticle diameter range in the water-absorbent resin compositions (11)to (17) as obtained from Examples 11 to 17 and in the comparativewater-absorbent resin composition (7) as obtained from ComparativeExample 7, was determined in the following way. The results are shown inTable 7.

(i) The water-absorbent resin composition was sieved with JIS standardsieves having mesh opening sizes of 600 μm, 425 μm, and 300 μm todetermine the distribution into the particle diameter ranges of 600/425μm, 425/300 μm, and 300 μm-pass.

(ii) An amount of 1 g of the water-absorbent resin composition, asclassified into each particle diameter range in the above step (i), wasprecisely weighed out.

(iii) A Teflon (registered trademark) rotator of 35 mm was placed into apolypropylene-made beaker of 260 ml., and then thereto 1 g of thewater-absorbent resin composition, as weighed out in the above step(ii), 190 g of 0.9 mass % aqueous sodium chloride solution, and 10 g of2N hydrochloric acid were added, and then they were stirred with amagnetic stirrer for 5 minutes.

(iv) After the stirring, the resultant supernatant was sucked up with apolypropylene-made syringe and then filtered with a chromatodisk (GLChromatodisk 25A, produced by GL Science).

(v) The filtrate was analyzed by ICP (plasma emission spectrometry) toquantify the amount (%) of the aluminum sulfate which was containedevery particle diameter range in the water-absorbent resin compositionas classified into each particle diameter range.

(vi) The distribution of the aluminum sulfate every particle diameterrange was determined in accordance with the following equation:Distribution (%) of aluminum sulfate every particle diameterrange=[amount (%) of aluminum sulfate every particle diameterrange×particle diameter distribution (%)]×100/Σ[amount (%) of aluminumsulfate every particle diameter range×particle diameter distribution(%)]

For example, when the particle diameter distribution in the range of600/425 μm is 13% and the amount of the aluminum sulfate in the range of600/425 μm is 0.20%, and when the particle diameter distribution in therange of 425/300 μm is 45% and the amount of the aluminum sulfate in therange of 425/300 μm is 0.17%, and when the particle diameterdistribution in the range of 300 μm-pass is 42% and the amount of thealuminum sulfate in the range of 300 μm-pass is 0.89%, then thedistribution (%) of the aluminum sulfate in the range of 600/425 μm isdetermined as follows:Distribution (%) of aluminum sulfate in the range of 600/425μm=[0.20×0.13]×100/(0.20×0.13+0.17×0.45+0.89×0.42)=5(%)

TABLE 5 Mass- average particle diameter CRC AAP SFC after PS (μm) (g/g)(g/g) (×10⁻⁷ · cm³ · s · g⁻¹) Water-absorbent resin (1D) 320 26 25 45Water-absorbent resin (2D) 430 29 24 35 Water-absorbent resin 325 26 24130 composition (11) Water-absorbent resin 323 26 24 120 composition(12) Water-absorbent resin 328 26 24 128 composition (13)Water-absorbent resin 321 26 24 125 composition (14) Water-absorbentresin 335 26 24 123 composition (15) Water-absorbent resin 336 26 24 121composition (16) Water-absorbent resin 340 26 24 122 composition (17)Water-absorbent resin 320 25 23 118 composition (18) Water-absorbentresin 326 26 24 160 composition (19) Water-absorbent resin 435 29 24 85composition (20) Comparative water-absorbent 320 26 25 120 resincomposition (7) Comparative water-absorbent 316 26 22 100 resincomposition (8) Comparative water-absorbent 335 26 21 45 resincomposition (9) Comparative water-absorbent 340 27 21 40 resincomposition (10)

TABLE 6 Dust generation degree (mg/m³) Water-absorbent resin 0.19composition (11) Water-absorbent resin 0.14 composition (12)Water-absorbent resin 0.14 composition (13) Water-absorbent resin 0.12composition (14) Water-absorbent resin (1D) 0.29

TABLE 7 Distribution of aluminum sulfate every particle diameter range600/425 μm 425/300 μm 300 μm-pass (%) (%) (%) Water-absorbent resin 4  987 composition (11) Water-absorbent resin 5 15 80 composition (12)Water-absorbent resin 5 15 80 composition (13) Water-absorbent resin 516 79 composition (14) Water-absorbent resin 6 23 71 composition (15)Water-absorbent resin 7 20 73 composition (16) Water-absorbent resin 11 23 66 composition (17) Comparative water-absorbent 2 10 88 resincomposition (7)

REFERENTIAL EXAMPLE 10

In a reactor as prepared by lidding a jacketed stainless twin-armkneader of 10 liters in capacity having two sigma-type blades, there wasprepared a reaction liquid by dissolving 7.14 g (0.06 mol %) ofpolyethylene glycol diacrylate into 5,438 g of aqueous solution ofsodium acrylate having a neutralization degree of 71.3 mol % (monomerconcentration: 39 mass %). Next, this reaction liquid was deaeratedunder an atmosphere of nitrogen gas for 30 minutes. Subsequently, 29.34g of 10 mass % aqueous sodium persulfate solution and 24.45 g of 0.1mass % aqueous L-ascorbic acid solution were added thereto under stirredconditions. As a result, polymerization started after about 1 minute.Then, the polymerization was carried out in the range of 20 to 95° C.while the forming gel was pulverized. Then, the resultant crosslinkedhydrogel polymer was taken out after 30 minutes from the start of thepolymerization.

The crosslinked hydrogel polymer as obtained above was in the form offinely divided pieces having diameters of not larger than about 5 mm.This finely divided crosslinked hydrogel polymer was spread onto a metalgauze of 50 meshes (mesh opening size: 300 μm) and then hot-air-dried at180° C. for 50 minutes. The resultant water-absorbent resin waspulverized with a roll mill and then further classified with JISstandard sieves having mesh opening sizes of 850 μm and 150 μm, thusobtaining water-absorbent resin particles (a).

REFERENTIAL EXAMPLE 11

The same polymerization operation as of Referential Example 10 wascarried out except that the reaction liquid was replaced with a reactionliquid as prepared by dissolving 4.02 g (0.035 mol %) of polyethyleneglycol diacrylate into 5,444 g of aqueous solution of sodium acrylatehaving a neutralization degree of 75 mol % (monomer concentration: 38mass %).

The resultant water-absorbent resin was pulverized with a roll mill andthen further classified with JIS standard sieves having mesh openingsizes of 710 μm and 150 μm, thus obtaining water-absorbent resinparticles (b).

REFERENTIAL EXAMPLE 12

The same polymerization operation as of Referential Example 10 wascarried out except that the reaction liquid was replaced with a reactionliquid as prepared by dissolving 11.7 g (0.1 mol %) of polyethyleneglycol diacrylate into 5,438 g of aqueous solution of sodium acrylatehaving a neutralization degree of 71 mol % (monomer concentration: 38mass %).

The resultant water-absorbent resin was pulverized with a roll mill andthen further classified with JIS standard sieves having mesh openingsizes of 600 μm and 150 μm, thus obtaining water-absorbent resinparticles (c).

REFERENTIAL EXAMPLE 13

An amount of 100 g of the water-absorbent resin (a) as obtained from theaforementioned Referential Example 10 was uniformly mixed with asurface-treating agent comprising a mixed liquid of 1.0 g of ethylenecarbonate and 3.0 g of pure water, and then the resultant mixture washeat-treated at 180° C. for 30 minutes. Furthermore, the resultantparticles were disintegrated to such a degree that they could passthrough a JIS standard sieve having a mesh opening size of 850 μm. As aresult, water-absorbent resin particles (a1) were obtained.

REFERENTIAL EXAMPLE 14

An amount of 100 g of the water-absorbent resin (b) as obtained from theaforementioned Referential Example 11 was uniformly mixed with asurface-treating agent comprising a mixed liquid of 1.0 g of2-oxazolidone and 3.0 g of pure water, and then the resultant mixturewas heat-treated at 185° C. for 30 minutes. Furthermore, the resultantparticles were disintegrated to such a degree that they could passthrough a JIS standard sieve having a mesh opening size of 710 μm. As aresult, water-absorbent resin particles (b1) were obtained.

REFERENTIAL EXAMPLE 15

An amount of 100 g of the water-absorbent resin particles (c) asobtained from the aforementioned Referential Example 12 were uniformlymixed with a surface-treating agent comprising a mixed liquid of 1.0 gof 1,4-butanediol and 3.0 g of pure water, and then the resultantmixture was heat-treated at 190° C. for 30 minutes. Furthermore, theresultant particles were disintegrated to such a degree that they couldpass through a JIS standard sieve having a mesh opening size of 600 μm.As a result, water-absorbent resin particles (c1) were obtained.

EXAMPLE 22

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were heated to140° C., and then mixed with 2 g of potassium alum (potassium aluminumsulfate dodecahydrate) under stirring, and then the stirring wascontinued for 10 minutes, thus obtaining a water-absorbent resincomposition (22).

COMPARATIVE EXAMPLE 11

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were mixed withan aqueous solution comprising 2 g of potassium alum (potassium aluminumsulfate dodecahydrate) and 8 g of water under stirring, thus obtaining acomparative water-absorbent resin composition (11).

COMPARATIVE EXAMPLE 12

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were mixed with2 g of potassium alum (potassium aluminum sulfate dodecahydrate), andthen thereto 3 g of water was added under stirring, thus obtaining acomparative water-absorbent resin composition (12).

EXAMPLE 23

An amount of 100 g of the water-absorbent resin particles (b1) asobtained from the aforementioned Referential Example 14 were mixed with1.5 g of ammonium alum (ammonium aluminum sulfate dodecahydrate) understirring, and then the resultant mixture was heated to 130° C., and thenthe stirring was continued for 15 minutes, thus obtaining awater-absorbent resin composition (23).

COMPARATIVE EXAMPLE 13

An amount of 100 g of the water-absorbent resin particles (b1) asobtained from the aforementioned Referential Example 14 were mixed withan aqueous solution comprising 1.5 g of ammonium alum (ammonium aluminumsulfate dodecahydrate) and 8.5 g of water under stirring, thus obtaininga comparative water-absorbent resin composition (13).

COMPARATIVE EXAMPLE 14

An amount of 100 g of the water-absorbent resin particles (b1) asobtained from the aforementioned Referential Example 14 were mixed with1.5 g of ammonium alum (ammonium aluminum sulfate dodecahydrate), andthen thereto 3 g of water was added under stirring, thus obtaining acomparative water-absorbent resin composition (14).

[As to Water-Absorbent Resin Compositions (22) to (23) and ComparativeWater-Absorbent Resin Compositions (11) to (14) as Obtained fromExamples 22 to 23 and Comparative Examples 11 to 14]:

Shown in Table 8 are the CRC, AAP, and SFC of the water-absorbent resincompositions (22) to (23) and comparative water-absorbent resincompositions (11) to (14) as obtained from Examples 22 to 23 andComparative Examples 11 to 14.

In addition, shown in FIGS. 5 and 6 are: views (FIG. 5-(a) and FIG.6-(a)) obtained by taking electron photomicrographs of thewater-absorbent resin composition (22) as obtained from Example 22; and,as to these photomicrographs, views (FIG. 5-(b) and FIG. 6-(b)) obtainedby taking X-ray image photomicrographs of the sulfur element by anSEM-EDS (Energy Dispersive X-ray Spectrometer), wherein the sulfurelement is originated from the sulfate ion as contained in the potassiumalum. The views obtained by taking the X-ray image photomicrographs ofthe sulfur element are more sensitive and easier to see than X-rayimages of the aluminum element, and thus have been used (the X-rayimages of the aluminum element also display the same distributions as ofthe sulfur element).

TABLE 8 Water- absorbent resin particles or water- absorbent resin MetalAddition CRC AAP SFC Example No. composition compound method (g/g) (g/g)(×10⁻⁷ · cm³ · s · g⁻¹) Referential Water- — — 26.3 23.7 73 Example 15absorbent resin particles (c1) Example 22 Water- Potassium Heat-fusion26.4 23.6 164 absorbent resin alum composition (22) ComparativeComparative Potassium Aqueous 25.3 20.0 101 Example 11 water-absorbentalum solution resin addition composition (11) Comparative ComparativePotassium Dry mixing + addition 25.8 22.9 120 Example 12 water-absorbentalum of resin water composition (12) Referential Water- — — 31.2 25.1 25Example 14 absorbent resin particles (b1) Example 23 Water- AmmoniumHeat-fusion 31.5 24.7 56 absorbent resin alum composition (23)Comparative Comparative Ammonium Aqueous 30.1 21.1 32 Example 13water-absorbent alum solution resin addition composition (13)Comparative Comparative Ammonium Dry mixing + addition 30.5 24.1 36Example 14 water-absorbent alum of resin water composition (14)

From Table 8, it can be understood that: even in the case where the sameamount of metal compound is added to the same water-absorbent resinparticles, the water-absorbent resin compositions according to thepresent invention have excellent liquid permeability and liquiddiffusibility, and also are water-absorbent resin compositions excellentin point of little deterioration of the CRC and AAP.

From FIGS. 5 and 6, there can be well seen a state where the metalcompound is fused to surfaces of the water-absorbent resin particles. Asis illustrated by FIG. 5, in the water-absorbent resin compositionaccording to the present invention, favorably, at least a part of themetal compound is fused in the form of coating at least a part ofsurfaces of the water-absorbent resin particles in a layered state. Inaddition, as is illustrated by FIG. 6, there is also preferred a formsuch that inter-particular binding between the water-absorbent resinparticles and particles of the metal compound is formed by the fusion.

EXAMPLE 24

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were heated to170° C., and then mixed with 1 g of aluminum sulfate tetradeca- tooctadecahydrates (mass-average particle diameter: 150 μm) under stirringfor 5 minutes, thus obtaining a water-absorbent resin composition (24).

COMPARATIVE EXAMPLE 15

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were heated to70° C., and then mixed with 1 g of aluminum sulfate tetradeca- tooctadecahydrates (mass-average particle diameter: 150 μm) under stirringfor 5 minutes, thus obtaining a comparative water-absorbent resincomposition (15).

COMPARATIVE EXAMPLE 16

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were mixed with1 g of aluminum sulfate tetradeca- to octadecahydrates (mass-averageparticle diameter: 150 μm) under stirring for 5 minutes, thus obtaininga comparative water-absorbent resin composition (16).

COMPARATIVE EXAMPLE 17

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were mixed with1 g of aluminum sulfate tetradeca- to octadecahydrates (mass-averageparticle diameter: 150 μm), and then thereto 3 g of water was addedunder stirring, thus obtaining a comparative water-absorbent resincomposition (17).

COMPARATIVE EXAMPLE 18

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were mixed withan aqueous solution comprising 1 g of aluminum sulfate tetradeca- tooctadecahydrates (mass-average particle diameter: 150 μm) and 4 g ofwater under stirring, thus obtaining a comparative water-absorbent resincomposition (18).

[As to Water-Absorbent Resin Composition (24) and ComparativeWater-Absorbent Resin Compositions (15) to (18) as Obtained from Example24 and Comparative Examples 15 to 18]:

Shown in Table 9 are the results of having measured the content of thealuminum sulfate tetradeca- to octadecahydrates (hereinafter abbreviatedto ASH) every particle diameter range in the water-absorbent resincompositions (22) and (24) and comparative water-absorbent resincompositions (15) to (18) as obtained from Examples 22 and 24 andComparative Examples 15 to 18. Incidentally, the results were calculatedfrom the aluminum contents as measured by the aforementioned plasmaemission spectrometry.

TABLE 9 Water- absorbent resin particles or water- absorbent resin AL AL(<300 μm) Metal compound Example No. composition Addition method ppm ppmsegregation index Referential Water- — — — — Example 15 absorbent resinparticles (c1) Example 22 Water- Heat-fusion 114 104 0.91 absorbentresin composition (22) Example 24 Water- Heat-fusion 91 137 1.51absorbent resin composition (24) Comparative Comparative Heating tolower 90 218 2.42 Example 15 water-absorbent than melting point resincomposition (15) Comparative Comparative Dry mixing 91 228 2.51 Example16 water-absorbent resin composition (16) Comparative Comparative Drymixing + addition 92 204 2.22 Example 17 water-absorbent of water resincomposition (17) Comparative Comparative Aqueous solution 90 143 1.59Example 18 water-absorbent addition resin composition (18) AL: aluminumcontent (ppm) of the water-absorbent resin composition or comparativewater-absorbent resin composition AL (<300 μm): aluminum content (ppm)of particles passing through a mesh having a mesh opening size of 300 μmin the water-absorbent resin composition or comparative water-absorbentresin composition

As can be understood from Table 9, because the ASH which is contained inthe water-absorbent resin composition (24) as obtained from Example 24is fused and entirely fixed to surfaces of water-absorbent resinparticles, the ASH content of the particles passing through the meshhaving a mesh opening size of 300 μm is low and almost the same as thatin the case where the ASH is added in the form of an aqueous solution(comparative water-absorbent resin composition (18)). In comparison, inthe cases of other methods such as dry mixing (comparativewater-absorbent resin compositions (15) to (17)), because the ASH havingfine particle diameters is not sufficiently fixed, the ASH content ofthe particles passing through the mesh having a mesh opening size of 300μm is unfavorably high. From these results, it can be understood thatthe water-absorbent resin compositions according to the presentinvention are water-absorbent resin compositions excellent in that thesegregation of the contained metal compound occurs very little. Inaddition, the process in which the aqueous solution of the metalcompound is added may be effective for the prevention of the segregationof the metal compound similarly to the process according to the presentinvention, but, as shown in Table 8, has demerits in that theSFC-enhancing effect is low and in that the deterioration of the AAP islarge.

EXAMPLE 25

An amount of 100 g of the water-absorbent resin particles (a1) asobtained from the aforementioned Referential Example 13 were mixed with0.8 g of aluminum nitrate nonahydrate under stirring, and then theresultant mixture was heated to 110° C., and then the stirring wascontinued for 10 minutes, thus obtaining a water-absorbent resincomposition (25).

COMPARATIVE EXAMPLE 19

An amount of 100 g of the water-absorbent resin particles (a1) asobtained from the aforementioned Referential Example 13 were mixed withan aqueous solution comprising 0.8 g of aluminum nitrate nonahydrate and7.2 g of water under stirring, thus obtaining a comparativewater-absorbent resin composition (19).

Comparative Example 20

An amount of 100 g of the water-absorbent resin particles (a1) asobtained from the aforementioned Referential Example 13 were mixed with0.8 g of aluminum nitrate nonahydrate, and then thereto an aqueoussolution comprising 3 g of water and 1 g of polyethylene glycol (averagemolecular weight: 300) was added under stirring, thus obtaining acomparative water-absorbent resin composition (20).

[As to Water-Absorbent Resin Composition (25) and ComparativeWater-Absorbent Resin Compositions (19) to (20) as Obtained from Example25 and Comparative Examples 19 to 20]:

Shown in Table 10 are the results of having measured the CRC, AAP, andBR of the water-absorbent resin composition (25) and comparativewater-absorbent resin compositions (19) to (20) as obtained from Example25 and Comparative Examples 19 to 20.

TABLE 10 Water- absorbent resin particles or water- absorbent resinMetal Addition CRC AAP Example No. composition compound method (g/g)(g/g) BR % Referential Water- — — 36.1 25.9 35.5 Example 13 absorbentresin particles (a1) Example 25 Water- ANH Heat-fusion 36.3 25.5 13.2absorbent resin composition (25) Comparative Comparative ANH Aqueous35.2 22.0 33.1 Example 19 water- solution absorbent resin additioncomposition (19) Comparative Comparative ANH Dry mixing + addition 35.624.1 21.7 Example 20 water- of absorbent resin water composition (20)ANH: aluminum nitrate nonahydrate

From Table 10, it can be understood that: even in the case where thesame amount of metal compound is added to the same water-absorbent resinparticles, the water-absorbent resin composition according to thepresent invention is a water-absorbent resin composition which undergoeslittle deterioration of the CRC and AAP and is excellent in the BR.

EXAMPLE 26

An amount of 100 g of the water-absorbent resin particles (b1) asobtained from the aforementioned Referential Example 14 were heated to120° C., and then mixed with 1 g of aluminum chloride hexahydrate understirring for 5 minutes, thus obtaining a water-absorbent resincomposition (26).

EXAMPLE 27

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were heated to100° C., and then mixed with 2 g of sodium aluminum sulfatedodecahydrate under stirring for 5 minutes, thus obtaining awater-absorbent resin composition (27).

COMPARATIVE EXAMPLE 21

An amount of 100 g of the water-absorbent resin particles (b1) asobtained from the aforementioned Referential Example 14 were heated to120° C., and then mixed with 1 g of paraffin wax (melting point: 83° C.)under stirring for 5 minutes, thus obtaining a comparativewater-absorbent resin composition (21).

COMPARATIVE EXAMPLE 22

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were heated to160° C., and then mixed with 1 g of zinc caprylate under stirring for 5minutes, thus obtaining a comparative water-absorbent resin composition(22).

COMPARATIVE EXAMPLE 23

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were uniformlymixed with 5 g of TRICLOSAN, and then the resultant mixture was heatedto 80° C., and then the mixing was continued under stirring for 1 hour,thus obtaining a comparative water-absorbent resin composition (23).

[As to Water-Absorbent Resin Compositions (26) to (27) and ComparativeWater-Absorbent Resin Compositions (21) to (23) as Obtained fromExamples 26 to 27 and Comparative Examples 21 to 23]:

Shown in Table 11 are the CRC, SFC, and CSF of the water-absorbent resincompositions (26) to (27) and comparative water-absorbent resincompositions (21) to (23) as obtained from Examples 26 to 27 andComparative Examples 21 to 23.

TABLE 11 Particulate water- absorbent resin or water- absorbent resinMetal Addition CRC SFC CSF Example No. composition compound method (g/g)(×10⁻⁷ · cm³ · s · g⁻¹) (g/g) Referential Water- — — 31.2 25 24.1Example 14 absorbent resin particles (b1) Example 23 Water- AmmoniumHeat-fusion 31.5 56 23.6 absorbent resin alum composition (23) Example26 Water- ACH Heat-fusion 31.2 51 23.5 absorbent resin composition (26)Comparative Comparative Paraffin wax Heat-fusion 31.0 24 12.1 Example 21water- absorbent resin composition (21) Referential Water- — — 26.3 7325.1 Example 15 absorbent resin particles (c1) Example 22 Water-Potassium Heat-fusion 26.4 164  22.4 absorbent resin alum composition(22) Example 27 Water- ASSH Heat-fusion 26.3 157  22.1 absorbent resincomposition (27) Comparative Comparative Zinc caprylate Heat-fusion 26.281 9.8 Example 22 water- absorbent resin composition (22) ComparativeComparative TRICLOSAN Heat-fusion 26.1 71 13.1 Example 23 water-absorbent resin composition (23) ACH: aluminum chloride hexahydrateASSH: sodium aluminum sulfate dodecahydrate Zinc caprylate:(CH₃(CH₂)₆COO)₂Zn TRICLOSAN: 2′,4′,4-trichloro-2-hydroxydiphenyl ether

From Table 11, it can be understood that the water-absorbent resincompositions according to the present invention have excellent SFC andCSF and are excellent in the liquid permeability and liquiddiffusibility and in the capillary suction force. As to such asheat-fusible resins like the comparative water-absorbent resincomposition (21), the effects of the present invention are not obtained,and, on the contrary, such as deterioration of the CSF is brought about.In the case where the polyvalent metal salt of the organic acid havingnot fewer than 7 carbon atoms per molecule is used like the case of thecomparative water-absorbent resin composition (22), great deteriorationof the CSF is caused. Also in the case of using the organic substancelike the case of the comparative water-absorbent resin composition (23),the effects of the present invention are not obtained, and, on thecontrary, such as deterioration of the CSF is brought about.

COMPARATIVE EXAMPLE 24

An amount of 100 g of the water-absorbent resin particles (c1) asobtained from the aforementioned Referential Example 15 were mixed with2 g of potassium alum (potassium aluminum sulfate dodecahydrate) understirring, and then the stirring was continued for 10 minutes, thusobtaining a comparative water-absorbent resin composition (24).

[As to Comparative Water-Absorbent Resin Composition (24) as Obtainedfrom Comparative Example 24]:

Shown in Table 12 are the BR of the water-absorbent resin composition(22) and comparative water-absorbent resin compositions (11), (12) and(24) as obtained from Example 22 and Comparative Examples 11, 12 and 24.

TABLE 12 Water- absorbent resin particles or water- absorbent resinMetal Addition BR Example No. composition compound method % ReferentialWater- — — 28.9 Example 15 absorbent resin particles (c1) Example 22Water- Potassium Heat-fusion  4.7 absorbent resin alum composition (22)Comparative Comparative Potassium Aqueous 25.1 Example 11water-absorbent alum solution resin addition composition (11)Comparative Comparative Potassium Dry mixing + 15.6 Example 12water-absorbent alum addition of resin water composition (12)Comparative Comparative Potassium Dry mixing 10.3 Example 24water-absorbent alum resin composition (24)

From Table 12, it can be understood that the water-absorbent resincomposition according to the present invention has a more excellent BRand is more excellent in the handling property during the moistureabsorption when compared with other addition methods.

Shown in Table 13 are the particle diameter distributions of thewater-absorbent resin particles (a1), (b1) and (c1) as obtained fromReferential Examples 13 to 15 and the water-absorbent resin compositions(22) to (27) as obtained from Examples 22 to 27.

TABLE 13 Referential Example No. and Example No. Referential ReferentialReferential Example Example Example Example Example Example ExampleExample Example 13 14 15 22 23 24 25 26 27 Water- (a1) (b1) (c1) — — — —— — absorbent resin particles Water- — — — (22) (23) (24) (25) (26) (27)absorbent resin composition ≧850 μm 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0(mass %) 850-710 μm 2.9 0.0 0.0 0.0 0.1 0.0 3.1 0.1 0.0 (mass %) 710-600μm 28.0 6.5 0.0 0.1 6.8 0.1 28.4 6.6 0.0 (mass %) 600-500 μm 17.1 8.93.1 3.4 9.3 3.3 17.5 9.2 3.3 (mass %) 500-425 μm 13.7 18.3 17.1 18.419.2 17.6 14.1 18.7 17.5 (mass %) 425-300 μm 20.7 37.3 35.4 35.8 37.134.4 21.5 37.4 35.3 (mass %) 300-212 μm 10.9 16.8 27.4 26.3 16.5 27.29.8 16.6 27.8 (mass %) 212-150 μm 4.2 7.3 13.1 12.5 6.9 13.2 3.5 7.112.9 (mass %) 150-45 μm 2.3 4.8 3.8 3.5 4.1 4.1 2.1 4.3 3.2 (mass %) ≦45μm 0.2 0.1 0.1 0.0 0.0 0.1 0.0 0.0 0.0 (mass %) Total 100.0 100.0 100.0100.0 100.0 100.0 100.0 100.0 100.0 (mass %) D50 (μm) 488 366 315 322372 315 494 369 317 σζ 0.40 0.38 0.37 0.37 0.37 0.38 0.37 0.37 0.36 (≧Aμm) represents water-absorbent resin particles or a water-absorbentresin composition remaining on a sieve of A in mesh opening size as aresult of the classification operation. (≦B μm) representswater-absorbent resin particles or a water-absorbent resin compositionhaving passed through a sieve of B in mesh opening size as a result ofthe classification operation. (A-B μm) represents water-absorbent resinparticles or a water-absorbent resin composition having passed throughthe sieve of A in mesh opening size and remaining on the sieve of B inmesh opening size as a result of the classification operation.

INDUSTRIAL APPLICATION

Because the water-absorbent resin compositions (1) and (2) according tothe present invention have excellent water absorption properties, thesewater-absorbent resin compositions can be used as water-absorbing andwater-retaining agents for various purposes. For example, thesewater-absorbent resin compositions can be used for such as:water-absorbing and water-retaining agents for absorbent articles (e.g.disposable diapers, sanitary napkins, incontinent pads, and medicalpads); agricultural and horticultural water-retaining agents (e.g.substitutes for peat moss, soil-modifying-and-improving agents,water-retaining agents, and agents for duration of effects ofagricultural chemicals); water-retaining agents for buildings (e.g.dew-condensation-preventing agents for interior wall materials, cementadditives); release control agents; coldness-retaining agents;disposable portable body warmers; sludge-solidifying agents;freshness-retaining agents for foods; ion-exchange column materials;dehydrating agents for sludge or oil; desiccating agents; andhumidity-adjusting materials. In addition, the water-absorbent resincompositions (1) and (2) according to the present invention can be usedparticularly favorably for sanitary materials for absorption ofexcrement, urine, or blood, such as disposable diapers and sanitarynapkins.

1. A water-absorbent resin composition, which is a water-absorbent resincomposition comprising water-absorbent resin particles obtained bypolymerizing a monomer including acrylic acid and/or its salt, with thecomposition having a mass-average particle diameter of 100 to 600 μm andcomprising water-soluble polyvalent metal salt particles and thewater-absorbent resin particles that have been surface-crosslinked, andwhere at least a part of the water-absorbent resin particles areagglomerates, and at least a part of the water-soluble polyvalent metalsalt particles are fused to surfaces of the water-absorbent resinparticles.
 2. A water-absorbent resin composition according to claim 1,wherein the water-soluble polyvalent metal salt particles are particlesof an aluminum salt having water of crystallization.
 3. Awater-absorbent resin composition according to claim 1, wherein thewater-absorbent resin particles are those which have beensurface-crosslinked with a polyhydric alcohol.
 4. A water-absorbentresin composition, which is a water-absorbent resin composition having amass-average particle diameter of 100 to 600 μM comprisingwater-absorbent resin particles and water-soluble polyvalent metal saltparticles, wherein the water-absorbent resin particles are obtained bypolymerizing a monomer including acrylic acid and/or its salt, with thecomposition having a saline flow conductivity of at least 50(×10⁻⁷·cm³·s·g⁻¹) and a retention ratio of the saline flow conductivityof not less than 40%, and where at least a part of the water-absorbentresin particles are agglomerates, and at least a part of thewater-soluble polyvalent metal salt particles are fused to surfaces ofthe water-absorbent resin particles.
 5. A water-absorbent resincomposition according to claim 4, wherein the retention ratio of thesaline flow conductivity after a paint shaker test is not less than 70%.6. A process for production of a water-absorbent resin composition,which is characterized by comprising the steps of: adding a binder towater-absorbent resin particles obtained by polymerizing a monomerincluding acrylic acid and/or its salt; and then mixing the binder andthe water-absorbent resin particles with water-soluble polyvalent metalsalt particles to produce the water-absorbent resin composition having amass-average particle diameter of 100 to 600 μm, and where at least apart of the water-absorbent resin particles are agglomerates, and atleast a part of the water-soluble polyvalent metal salt particles arefused to surfaces of the water-absorbent resin particles.
 7. A processfor production of a water-absorbent resin composition according to claim6, wherein the water-absorbent resin particles are surface-crosslinkedones.
 8. A process for production of a water-absorbent resin compositionaccording to claim 6, wherein the binder contains a surface-crosslinkingagent.
 9. A process for production of a water-absorbent resincomposition according to claim 6, wherein the binder includes waterand/or a polyhydric alcohol.
 10. A process for production of awater-absorbent resin composition according to claim 6, wherein, whenthe binder is added to the water-absorbent resin particles, thetemperature of the water-absorbent resin particles is in the range of 40to 100° C.
 11. A water-absorbent resin composition according to claim 5,wherein the water-absorbent resin particles have a mass-average particlediameter of not larger than 500 μm; and the water-soluble polyvalentmetal salt has a mass-average particle diameter of not larger than 500μm.
 12. A water-absorbent resin composition according to claim 1, 4 or5, wherein the water-absorbent resin particles have beensurface-crosslinked with an organic surface-crosslinking agent.
 13. Awater-absorbent resin composition according to claim 12, wherein theorganic surface-crosslinking agent is a polyhydric alcohol.
 14. Aprocess for production of a water-absorbent resin composition accordingto claim 6, 7 or 8, wherein the water-absorbent resin particles have amass-average particle diameter of not larger than 500 μm; and thewater-soluble polyvalent metal salt particles have a mass-averageparticle diameter of not larger than 500 μm.
 15. A water-absorbent resincomposition according to claim 4, wherein the composition is prepared bydry mixing the water-soluble polyvalent metal salt and thewater-absorbent resin particles.
 16. A water-absorbent resin compositionaccording to claim 1 or claim 4, wherein the water-soluble polyvalentmetal salt particles are a powder having a mass-average particlediameter of not larger than 1,000 μm.
 17. A water-absorbent resincomposition according to claim 1, wherein the fusion is heat-fusion. 18.A water-absorbent resin composition according to claim 4, which has asaline flow conductivity of not less than 100×10⁻⁷·cm³·s/g.
 19. Theprocess for production of a water-absorbent resin composition accordingto claim 6, wherein the water-soluble polyvalent metal salt particlescomprise one or more members selected from the group consisting ofalkaline metal salts and polyvalent metal salts (except polyvalent metalsalts of organic acids having not fewer than 7 carbon atoms permolecule); with the process comprising the steps of: heating thewater-absorbent resin particles and/or the water-soluble polyvalentmetal salt particles to a temperature of not lower than the meltingpoint of the water-soluble polyvalent metal salt particles; and therebyfusing at least a part of the water-soluble polyvalent metal saltparticles to surfaces of the water-absorbent resin particles.
 20. Thewater-absorbent resin composition of claim 1, wherein the amount of theagglomerates is not smaller than 5 mass %.
 21. The water-absorbent resincomposition of claim 4, wherein the amount of the agglomerates is notsmaller than 5 mass %.
 22. The process for the production of awater-absorbent composition of claim 6, wherein the amount of theagglomerates is not smaller than 5 mass %.
 23. The water-absorbent resincomposition of claim 4, wherein the fusion is heat-fusion.
 24. Theprocess of claim 6, wherein the fusion is heat-fusion.